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• How to Write a Literature Review | Guide, Examples, & Templates

How to Write a Literature Review | Guide, Examples, & Templates

Published on January 2, 2023 by Shona McCombes . Revised on September 11, 2023.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research that you can later apply to your paper, thesis, or dissertation topic .

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates, and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarize sources—it analyzes, synthesizes , and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

What is the purpose of a literature review, examples of literature reviews, step 1 – search for relevant literature, step 2 – evaluate and select sources, step 3 – identify themes, debates, and gaps, step 4 – outline your literature review’s structure, step 5 – write your literature review, free lecture slides, other interesting articles, frequently asked questions, introduction.

• Quick Run-through
• Step 1 & 2

When you write a thesis , dissertation , or research paper , you will likely have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and its scholarly context
• Develop a theoretical framework and methodology for your research
• Position your work in relation to other researchers and theorists
• Show how your research addresses a gap or contributes to a debate
• Evaluate the current state of research and demonstrate your knowledge of the scholarly debates around your topic.

Writing literature reviews is a particularly important skill if you want to apply for graduate school or pursue a career in research. We’ve written a step-by-step guide that you can follow below.

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Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research problem and questions .

Make a list of keywords

Start by creating a list of keywords related to your research question. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list as you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some useful databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can also use boolean operators to help narrow down your search.

Make sure to read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

You likely won’t be able to read absolutely everything that has been written on your topic, so it will be necessary to evaluate which sources are most relevant to your research question.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models, and methods?
• Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible , and make sure you read any landmark studies and major theories in your field of research.

You can use our template to summarize and evaluate sources you’re thinking about using. Click on either button below to download.

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It is important to keep track of your sources with citations to avoid plagiarism . It can be helpful to make an annotated bibliography , where you compile full citation information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

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To begin organizing your literature review’s argument and structure, be sure you understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly visual platforms like Instagram and Snapchat—this is a gap that you could address in your own research.

There are various approaches to organizing the body of a literature review. Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarizing sources in order.

Try to analyze patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organize your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text , your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, you can follow these tips:

• Summarize and synthesize: give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: don’t just paraphrase other researchers — add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically evaluate: mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: use transition words and topic sentences to draw connections, comparisons and contrasts

In the conclusion, you should summarize the key findings you have taken from the literature and emphasize their significance.

When you’ve finished writing and revising your literature review, don’t forget to proofread thoroughly before submitting. Not a language expert? Check out Scribbr’s professional proofreading services !

This article has been adapted into lecture slides that you can use to teach your students about writing a literature review.

Scribbr slides are free to use, customize, and distribute for educational purposes.

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If you want to know more about the research process , methodology , research bias , or statistics , make sure to check out some of our other articles with explanations and examples.

• Sampling methods
• Simple random sampling
• Stratified sampling
• Cluster sampling
• Likert scales
• Reproducibility

 Statistics

• Null hypothesis
• Statistical power
• Probability distribution
• Effect size
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Research bias

• Optimism bias
• Cognitive bias
• Implicit bias
• Hawthorne effect
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• Explicit bias

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a thesis, dissertation , or research paper , in order to situate your work in relation to existing knowledge.

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarize yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your thesis or dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

A literature review is a survey of credible sources on a topic, often used in dissertations , theses, and research papers . Literature reviews give an overview of knowledge on a subject, helping you identify relevant theories and methods, as well as gaps in existing research. Literature reviews are set up similarly to other  academic texts , with an introduction , a main body, and a conclusion .

An  annotated bibliography is a list of  source references that has a short description (called an annotation ) for each of the sources. It is often assigned as part of the research process for a  paper .  

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What is a Literature Review? How to Write It (with Examples)

A literature review is a critical analysis and synthesis of existing research on a particular topic. It provides an overview of the current state of knowledge, identifies gaps, and highlights key findings in the literature. 1 The purpose of a literature review is to situate your own research within the context of existing scholarship, demonstrating your understanding of the topic and showing how your work contributes to the ongoing conversation in the field. Learning how to write a literature review is a critical tool for successful research. Your ability to summarize and synthesize prior research pertaining to a certain topic demonstrates your grasp on the topic of study, and assists in the learning process. 

Table of Contents

• What is the purpose of literature review? 
• a. Habitat Loss and Species Extinction: 
• b. Range Shifts and Phenological Changes: 
• c. Ocean Acidification and Coral Reefs: 
• d. Adaptive Strategies and Conservation Efforts: 

How to write a good literature review 

• Choose a Topic and Define the Research Question: 
• Decide on the Scope of Your Review: 
• Select Databases for Searches: 
• Conduct Searches and Keep Track: 
• Review the Literature: 
• Organize and Write Your Literature Review: 
• How to write a literature review faster with Paperpal? 
• Frequently asked questions 

What is a literature review?

A well-conducted literature review demonstrates the researcher’s familiarity with the existing literature, establishes the context for their own research, and contributes to scholarly conversations on the topic. One of the purposes of a literature review is also to help researchers avoid duplicating previous work and ensure that their research is informed by and builds upon the existing body of knowledge.

What is the purpose of literature review?

A literature review serves several important purposes within academic and research contexts. Here are some key objectives and functions of a literature review: 2  

1. Contextualizing the Research Problem: The literature review provides a background and context for the research problem under investigation. It helps to situate the study within the existing body of knowledge. 

2. Identifying Gaps in Knowledge: By identifying gaps, contradictions, or areas requiring further research, the researcher can shape the research question and justify the significance of the study. This is crucial for ensuring that the new research contributes something novel to the field. 

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3. Understanding Theoretical and Conceptual Frameworks: Literature reviews help researchers gain an understanding of the theoretical and conceptual frameworks used in previous studies. This aids in the development of a theoretical framework for the current research. 

4. Providing Methodological Insights: Another purpose of literature reviews is that it allows researchers to learn about the methodologies employed in previous studies. This can help in choosing appropriate research methods for the current study and avoiding pitfalls that others may have encountered. 

5. Establishing Credibility: A well-conducted literature review demonstrates the researcher’s familiarity with existing scholarship, establishing their credibility and expertise in the field. It also helps in building a solid foundation for the new research. 

6. Informing Hypotheses or Research Questions: The literature review guides the formulation of hypotheses or research questions by highlighting relevant findings and areas of uncertainty in existing literature. 

Literature review example

Let’s delve deeper with a literature review example: Let’s say your literature review is about the impact of climate change on biodiversity. You might format your literature review into sections such as the effects of climate change on habitat loss and species extinction, phenological changes, and marine biodiversity. Each section would then summarize and analyze relevant studies in those areas, highlighting key findings and identifying gaps in the research. The review would conclude by emphasizing the need for further research on specific aspects of the relationship between climate change and biodiversity. The following literature review template provides a glimpse into the recommended literature review structure and content, demonstrating how research findings are organized around specific themes within a broader topic. 

Literature Review on Climate Change Impacts on Biodiversity:

Climate change is a global phenomenon with far-reaching consequences, including significant impacts on biodiversity. This literature review synthesizes key findings from various studies: 

a. Habitat Loss and Species Extinction:

Climate change-induced alterations in temperature and precipitation patterns contribute to habitat loss, affecting numerous species (Thomas et al., 2004). The review discusses how these changes increase the risk of extinction, particularly for species with specific habitat requirements. 

b. Range Shifts and Phenological Changes:

Observations of range shifts and changes in the timing of biological events (phenology) are documented in response to changing climatic conditions (Parmesan & Yohe, 2003). These shifts affect ecosystems and may lead to mismatches between species and their resources. 

c. Ocean Acidification and Coral Reefs:

The review explores the impact of climate change on marine biodiversity, emphasizing ocean acidification’s threat to coral reefs (Hoegh-Guldberg et al., 2007). Changes in pH levels negatively affect coral calcification, disrupting the delicate balance of marine ecosystems. 

d. Adaptive Strategies and Conservation Efforts:

Recognizing the urgency of the situation, the literature review discusses various adaptive strategies adopted by species and conservation efforts aimed at mitigating the impacts of climate change on biodiversity (Hannah et al., 2007). It emphasizes the importance of interdisciplinary approaches for effective conservation planning. 

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Writing a literature review involves summarizing and synthesizing existing research on a particular topic. A good literature review format should include the following elements. 

Introduction: The introduction sets the stage for your literature review, providing context and introducing the main focus of your review. 

• Opening Statement: Begin with a general statement about the broader topic and its significance in the field. 
• Scope and Purpose: Clearly define the scope of your literature review. Explain the specific research question or objective you aim to address. 
• Organizational Framework: Briefly outline the structure of your literature review, indicating how you will categorize and discuss the existing research. 
• Significance of the Study: Highlight why your literature review is important and how it contributes to the understanding of the chosen topic. 
• Thesis Statement: Conclude the introduction with a concise thesis statement that outlines the main argument or perspective you will develop in the body of the literature review. 

Body: The body of the literature review is where you provide a comprehensive analysis of existing literature, grouping studies based on themes, methodologies, or other relevant criteria. 

• Organize by Theme or Concept: Group studies that share common themes, concepts, or methodologies. Discuss each theme or concept in detail, summarizing key findings and identifying gaps or areas of disagreement. 
• Critical Analysis: Evaluate the strengths and weaknesses of each study. Discuss the methodologies used, the quality of evidence, and the overall contribution of each work to the understanding of the topic. 
• Synthesis of Findings: Synthesize the information from different studies to highlight trends, patterns, or areas of consensus in the literature. 
• Identification of Gaps: Discuss any gaps or limitations in the existing research and explain how your review contributes to filling these gaps. 
• Transition between Sections: Provide smooth transitions between different themes or concepts to maintain the flow of your literature review. 

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Conclusion: The conclusion of your literature review should summarize the main findings, highlight the contributions of the review, and suggest avenues for future research. 

• Summary of Key Findings: Recap the main findings from the literature and restate how they contribute to your research question or objective. 
• Contributions to the Field: Discuss the overall contribution of your literature review to the existing knowledge in the field. 
• Implications and Applications: Explore the practical implications of the findings and suggest how they might impact future research or practice. 
• Recommendations for Future Research: Identify areas that require further investigation and propose potential directions for future research in the field. 
• Final Thoughts: Conclude with a final reflection on the importance of your literature review and its relevance to the broader academic community. 

Conducting a literature review

Conducting a literature review is an essential step in research that involves reviewing and analyzing existing literature on a specific topic. It’s important to know how to do a literature review effectively, so here are the steps to follow: 1  

Choose a Topic and Define the Research Question:

• Select a topic that is relevant to your field of study. 
• Clearly define your research question or objective. Determine what specific aspect of the topic do you want to explore? 

Decide on the Scope of Your Review:

• Determine the timeframe for your literature review. Are you focusing on recent developments, or do you want a historical overview? 
• Consider the geographical scope. Is your review global, or are you focusing on a specific region? 
• Define the inclusion and exclusion criteria. What types of sources will you include? Are there specific types of studies or publications you will exclude? 

Select Databases for Searches:

• Identify relevant databases for your field. Examples include PubMed, IEEE Xplore, Scopus, Web of Science, and Google Scholar. 
• Consider searching in library catalogs, institutional repositories, and specialized databases related to your topic. 

Conduct Searches and Keep Track:

• Develop a systematic search strategy using keywords, Boolean operators (AND, OR, NOT), and other search techniques. 
• Record and document your search strategy for transparency and replicability. 
• Keep track of the articles, including publication details, abstracts, and links. Use citation management tools like EndNote, Zotero, or Mendeley to organize your references. 

Review the Literature:

• Evaluate the relevance and quality of each source. Consider the methodology, sample size, and results of studies. 
• Organize the literature by themes or key concepts. Identify patterns, trends, and gaps in the existing research. 
• Summarize key findings and arguments from each source. Compare and contrast different perspectives. 
• Identify areas where there is a consensus in the literature and where there are conflicting opinions. 
• Provide critical analysis and synthesis of the literature. What are the strengths and weaknesses of existing research? 

Organize and Write Your Literature Review:

• Literature review outline should be based on themes, chronological order, or methodological approaches. 
• Write a clear and coherent narrative that synthesizes the information gathered. 
• Use proper citations for each source and ensure consistency in your citation style (APA, MLA, Chicago, etc.). 
• Conclude your literature review by summarizing key findings, identifying gaps, and suggesting areas for future research. 

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How to write a literature review faster with Paperpal?

Paperpal, an AI writing assistant, integrates powerful academic search capabilities within its writing platform. With the Research feature, you get 100% factual insights, with citations backed by 250M+ verified research articles, directly within your writing interface with the option to save relevant references in your Citation Library. By eliminating the need to switch tabs to find answers to all your research questions, Paperpal saves time and helps you stay focused on your writing.   

Here’s how to use the Research feature:  

• Ask a question: Get started with a new document on paperpal.com. Click on the “Research” feature and type your question in plain English. Paperpal will scour over 250 million research articles, including conference papers and preprints, to provide you with accurate insights and citations. 
• Review and Save: Paperpal summarizes the information, while citing sources and listing relevant reads. You can quickly scan the results to identify relevant references and save these directly to your built-in citations library for later access. 
• Cite with Confidence: Paperpal makes it easy to incorporate relevant citations and references into your writing, ensuring your arguments are well-supported by credible sources. This translates to a polished, well-researched literature review. 

The literature review sample and detailed advice on writing and conducting a review will help you produce a well-structured report. But remember that a good literature review is an ongoing process, and it may be necessary to revisit and update it as your research progresses. By combining effortless research with an easy citation process, Paperpal Research streamlines the literature review process and empowers you to write faster and with more confidence. Try Paperpal Research now and see for yourself.  

Frequently asked questions

A literature review is a critical and comprehensive analysis of existing literature (published and unpublished works) on a specific topic or research question and provides a synthesis of the current state of knowledge in a particular field. A well-conducted literature review is crucial for researchers to build upon existing knowledge, avoid duplication of efforts, and contribute to the advancement of their field. It also helps researchers situate their work within a broader context and facilitates the development of a sound theoretical and conceptual framework for their studies.

Literature review is a crucial component of research writing, providing a solid background for a research paper’s investigation. The aim is to keep professionals up to date by providing an understanding of ongoing developments within a specific field, including research methods, and experimental techniques used in that field, and present that knowledge in the form of a written report. Also, the depth and breadth of the literature review emphasizes the credibility of the scholar in his or her field.  

Before writing a literature review, it’s essential to undertake several preparatory steps to ensure that your review is well-researched, organized, and focused. This includes choosing a topic of general interest to you and doing exploratory research on that topic, writing an annotated bibliography, and noting major points, especially those that relate to the position you have taken on the topic. 

Literature reviews and academic research papers are essential components of scholarly work but serve different purposes within the academic realm. 3 A literature review aims to provide a foundation for understanding the current state of research on a particular topic, identify gaps or controversies, and lay the groundwork for future research. Therefore, it draws heavily from existing academic sources, including books, journal articles, and other scholarly publications. In contrast, an academic research paper aims to present new knowledge, contribute to the academic discourse, and advance the understanding of a specific research question. Therefore, it involves a mix of existing literature (in the introduction and literature review sections) and original data or findings obtained through research methods. 

Literature reviews are essential components of academic and research papers, and various strategies can be employed to conduct them effectively. If you want to know how to write a literature review for a research paper, here are four common approaches that are often used by researchers.  Chronological Review: This strategy involves organizing the literature based on the chronological order of publication. It helps to trace the development of a topic over time, showing how ideas, theories, and research have evolved.  Thematic Review: Thematic reviews focus on identifying and analyzing themes or topics that cut across different studies. Instead of organizing the literature chronologically, it is grouped by key themes or concepts, allowing for a comprehensive exploration of various aspects of the topic.  Methodological Review: This strategy involves organizing the literature based on the research methods employed in different studies. It helps to highlight the strengths and weaknesses of various methodologies and allows the reader to evaluate the reliability and validity of the research findings.  Theoretical Review: A theoretical review examines the literature based on the theoretical frameworks used in different studies. This approach helps to identify the key theories that have been applied to the topic and assess their contributions to the understanding of the subject.  It’s important to note that these strategies are not mutually exclusive, and a literature review may combine elements of more than one approach. The choice of strategy depends on the research question, the nature of the literature available, and the goals of the review. Additionally, other strategies, such as integrative reviews or systematic reviews, may be employed depending on the specific requirements of the research.

The literature review format can vary depending on the specific publication guidelines. However, there are some common elements and structures that are often followed. Here is a general guideline for the format of a literature review:  Introduction:   Provide an overview of the topic.  Define the scope and purpose of the literature review.  State the research question or objective.  Body:   Organize the literature by themes, concepts, or chronology.  Critically analyze and evaluate each source.  Discuss the strengths and weaknesses of the studies.  Highlight any methodological limitations or biases.  Identify patterns, connections, or contradictions in the existing research.  Conclusion:   Summarize the key points discussed in the literature review.  Highlight the research gap.  Address the research question or objective stated in the introduction.  Highlight the contributions of the review and suggest directions for future research.

Both annotated bibliographies and literature reviews involve the examination of scholarly sources. While annotated bibliographies focus on individual sources with brief annotations, literature reviews provide a more in-depth, integrated, and comprehensive analysis of existing literature on a specific topic. The key differences are as follows: 

References 

• Denney, A. S., & Tewksbury, R. (2013). How to write a literature review.  Journal of criminal justice education ,  24 (2), 218-234. 
• Pan, M. L. (2016).  Preparing literature reviews: Qualitative and quantitative approaches . Taylor & Francis. 
• Cantero, C. (2019). How to write a literature review.  San José State University Writing Center . 

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Libraries | Research Guides

Literature reviews, what is a literature review, learning more about how to do a literature review.

• Planning the Review
• The Research Question
• Choosing Where to Search
• Organizing the Review
• Writing the Review

A literature review is a review and synthesis of existing research on a topic or research question. A literature review is meant to analyze the scholarly literature, make connections across writings and identify strengths, weaknesses, trends, and missing conversations. A literature review should address different aspects of a topic as it relates to your research question. A literature review goes beyond a description or summary of the literature you have read. 

• Sage Research Methods Core Collection This link opens in a new window SAGE Research Methods supports research at all levels by providing material to guide users through every step of the research process. SAGE Research Methods is the ultimate methods library with more than 1000 books, reference works, journal articles, and instructional videos by world-leading academics from across the social sciences, including the largest collection of qualitative methods books available online from any scholarly publisher. – Publisher

• Next: Planning the Review >>
• Last Updated: May 2, 2024 10:39 AM
• URL: https://libguides.northwestern.edu/literaturereviews

• University of Texas Libraries

Literature Reviews

• What is a literature review?
• Steps in the Literature Review Process
• Define your research question
• Determine inclusion and exclusion criteria
• Choose databases and search
• Review Results
• Synthesize Results
• Analyze Results
• Librarian Support
• Artificial Intelligence (AI) Tools

What is a Literature Review?

A literature or narrative review is a comprehensive review and analysis of the published literature on a specific topic or research question. The literature that is reviewed contains: books, articles, academic articles, conference proceedings, association papers, and dissertations. It contains the most pertinent studies and points to important past and current research and practices. It provides background and context, and shows how your research will contribute to the field. 

A literature review should: 

• Provide a comprehensive and updated review of the literature;
• Explain why this review has taken place;
• Articulate a position or hypothesis;
• Acknowledge and account for conflicting and corroborating points of view

From  S age Research Methods

Purpose of a Literature Review

A literature review can be written as an introduction to a study to:

• Demonstrate how a study fills a gap in research
• Compare a study with other research that's been done

Or it can be a separate work (a research article on its own) which:

• Organizes or describes a topic
• Describes variables within a particular issue/problem

Limitations of a Literature Review

Some of the limitations of a literature review are:

• It's a snapshot in time. Unlike other reviews, this one has beginning, a middle and an end. There may be future developments that could make your work less relevant.
• It may be too focused. Some niche studies may miss the bigger picture.
• It can be difficult to be comprehensive. There is no way to make sure all the literature on a topic was considered.
• It is easy to be biased if you stick to top tier journals. There may be other places where people are publishing exemplary research. Look to open access publications and conferences to reflect a more inclusive collection. Also, make sure to include opposing views (and not just supporting evidence).

Source: Grant, Maria J., and Andrew Booth. “A Typology of Reviews: An Analysis of 14 Review Types and Associated Methodologies.” Health Information & Libraries Journal, vol. 26, no. 2, June 2009, pp. 91–108. Wiley Online Library, doi:10.1111/j.1471-1842.2009.00848.x.

Meryl Brodsky : Communication and Information Studies

Hannah Chapman Tripp : Biology, Neuroscience

Carolyn Cunningham : Human Development & Family Sciences, Psychology, Sociology

Larayne Dallas : Engineering

Janelle Hedstrom : Special Education, Curriculum & Instruction, Ed Leadership & Policy ​

Susan Macicak : Linguistics

Imelda Vetter : Dell Medical School

For help in other subject areas, please see the guide to library specialists by subject .

Periodically, UT Libraries runs a workshop covering the basics and library support for literature reviews. While we try to offer these once per academic year, we find providing the recording to be helpful to community members who have missed the session. Following is the most recent recording of the workshop, Conducting a Literature Review. To view the recording, a UT login is required.

• October 26, 2022 recording
• Last Updated: Jun 18, 2024 1:00 PM
• URL: https://guides.lib.utexas.edu/literaturereviews

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Writing a Literature Review

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A literature review is a document or section of a document that collects key sources on a topic and discusses those sources in conversation with each other (also called synthesis ). The lit review is an important genre in many disciplines, not just literature (i.e., the study of works of literature such as novels and plays). When we say “literature review” or refer to “the literature,” we are talking about the research ( scholarship ) in a given field. You will often see the terms “the research,” “the scholarship,” and “the literature” used mostly interchangeably.

Where, when, and why would I write a lit review?

There are a number of different situations where you might write a literature review, each with slightly different expectations; different disciplines, too, have field-specific expectations for what a literature review is and does. For instance, in the humanities, authors might include more overt argumentation and interpretation of source material in their literature reviews, whereas in the sciences, authors are more likely to report study designs and results in their literature reviews; these differences reflect these disciplines’ purposes and conventions in scholarship. You should always look at examples from your own discipline and talk to professors or mentors in your field to be sure you understand your discipline’s conventions, for literature reviews as well as for any other genre.

A literature review can be a part of a research paper or scholarly article, usually falling after the introduction and before the research methods sections. In these cases, the lit review just needs to cover scholarship that is important to the issue you are writing about; sometimes it will also cover key sources that informed your research methodology.

Lit reviews can also be standalone pieces, either as assignments in a class or as publications. In a class, a lit review may be assigned to help students familiarize themselves with a topic and with scholarship in their field, get an idea of the other researchers working on the topic they’re interested in, find gaps in existing research in order to propose new projects, and/or develop a theoretical framework and methodology for later research. As a publication, a lit review usually is meant to help make other scholars’ lives easier by collecting and summarizing, synthesizing, and analyzing existing research on a topic. This can be especially helpful for students or scholars getting into a new research area, or for directing an entire community of scholars toward questions that have not yet been answered.

What are the parts of a lit review?

Most lit reviews use a basic introduction-body-conclusion structure; if your lit review is part of a larger paper, the introduction and conclusion pieces may be just a few sentences while you focus most of your attention on the body. If your lit review is a standalone piece, the introduction and conclusion take up more space and give you a place to discuss your goals, research methods, and conclusions separately from where you discuss the literature itself.

Introduction:

• An introductory paragraph that explains what your working topic and thesis is
• A forecast of key topics or texts that will appear in the review
• Potentially, a description of how you found sources and how you analyzed them for inclusion and discussion in the review (more often found in published, standalone literature reviews than in lit review sections in an article or research paper)
• Summarize and synthesize: Give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: Don’t just paraphrase other researchers – add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically Evaluate: Mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: Use transition words and topic sentence to draw connections, comparisons, and contrasts.

Conclusion:

• Summarize the key findings you have taken from the literature and emphasize their significance
• Connect it back to your primary research question

How should I organize my lit review?

Lit reviews can take many different organizational patterns depending on what you are trying to accomplish with the review. Here are some examples:

• Chronological : The simplest approach is to trace the development of the topic over time, which helps familiarize the audience with the topic (for instance if you are introducing something that is not commonly known in your field). If you choose this strategy, be careful to avoid simply listing and summarizing sources in order. Try to analyze the patterns, turning points, and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred (as mentioned previously, this may not be appropriate in your discipline — check with a teacher or mentor if you’re unsure).
• Thematic : If you have found some recurring central themes that you will continue working with throughout your piece, you can organize your literature review into subsections that address different aspects of the topic. For example, if you are reviewing literature about women and religion, key themes can include the role of women in churches and the religious attitude towards women.
• Qualitative versus quantitative research
• Empirical versus theoretical scholarship
• Divide the research by sociological, historical, or cultural sources
• Theoretical : In many humanities articles, the literature review is the foundation for the theoretical framework. You can use it to discuss various theories, models, and definitions of key concepts. You can argue for the relevance of a specific theoretical approach or combine various theorical concepts to create a framework for your research.

What are some strategies or tips I can use while writing my lit review?

Any lit review is only as good as the research it discusses; make sure your sources are well-chosen and your research is thorough. Don’t be afraid to do more research if you discover a new thread as you’re writing. More info on the research process is available in our "Conducting Research" resources .

As you’re doing your research, create an annotated bibliography ( see our page on the this type of document ). Much of the information used in an annotated bibliography can be used also in a literature review, so you’ll be not only partially drafting your lit review as you research, but also developing your sense of the larger conversation going on among scholars, professionals, and any other stakeholders in your topic.

Usually you will need to synthesize research rather than just summarizing it. This means drawing connections between sources to create a picture of the scholarly conversation on a topic over time. Many student writers struggle to synthesize because they feel they don’t have anything to add to the scholars they are citing; here are some strategies to help you:

• It often helps to remember that the point of these kinds of syntheses is to show your readers how you understand your research, to help them read the rest of your paper.
• Writing teachers often say synthesis is like hosting a dinner party: imagine all your sources are together in a room, discussing your topic. What are they saying to each other?
• Look at the in-text citations in each paragraph. Are you citing just one source for each paragraph? This usually indicates summary only. When you have multiple sources cited in a paragraph, you are more likely to be synthesizing them (not always, but often
• Read more about synthesis here.

The most interesting literature reviews are often written as arguments (again, as mentioned at the beginning of the page, this is discipline-specific and doesn’t work for all situations). Often, the literature review is where you can establish your research as filling a particular gap or as relevant in a particular way. You have some chance to do this in your introduction in an article, but the literature review section gives a more extended opportunity to establish the conversation in the way you would like your readers to see it. You can choose the intellectual lineage you would like to be part of and whose definitions matter most to your thinking (mostly humanities-specific, but this goes for sciences as well). In addressing these points, you argue for your place in the conversation, which tends to make the lit review more compelling than a simple reporting of other sources.

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• Knowledge Base
• Dissertation
• What is a Literature Review? | Guide, Template, & Examples

What is a Literature Review? | Guide, Template, & Examples

Published on 22 February 2022 by Shona McCombes . Revised on 7 June 2022.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research.

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarise sources – it analyses, synthesises, and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

Why write a literature review, examples of literature reviews, step 1: search for relevant literature, step 2: evaluate and select sources, step 3: identify themes, debates and gaps, step 4: outline your literature review’s structure, step 5: write your literature review, frequently asked questions about literature reviews, introduction.

• Quick Run-through
• Step 1 & 2

When you write a dissertation or thesis, you will have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and scholarly context
• Develop a theoretical framework and methodology for your research
• Position yourself in relation to other researchers and theorists
• Show how your dissertation addresses a gap or contributes to a debate

You might also have to write a literature review as a stand-alone assignment. In this case, the purpose is to evaluate the current state of research and demonstrate your knowledge of scholarly debates around a topic.

The content will look slightly different in each case, but the process of conducting a literature review follows the same steps. We’ve written a step-by-step guide that you can follow below.

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Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research objectives and questions .

If you are writing a literature review as a stand-alone assignment, you will have to choose a focus and develop a central question to direct your search. Unlike a dissertation research question, this question has to be answerable without collecting original data. You should be able to answer it based only on a review of existing publications.

Make a list of keywords

Start by creating a list of keywords related to your research topic. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list if you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can use boolean operators to help narrow down your search:

Read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

To identify the most important publications on your topic, take note of recurring citations. If the same authors, books or articles keep appearing in your reading, make sure to seek them out.

You probably won’t be able to read absolutely everything that has been written on the topic – you’ll have to evaluate which sources are most relevant to your questions.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models and methods? Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• How does the publication contribute to your understanding of the topic? What are its key insights and arguments?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible, and make sure you read any landmark studies and major theories in your field of research.

You can find out how many times an article has been cited on Google Scholar – a high citation count means the article has been influential in the field, and should certainly be included in your literature review.

The scope of your review will depend on your topic and discipline: in the sciences you usually only review recent literature, but in the humanities you might take a long historical perspective (for example, to trace how a concept has changed in meaning over time).

Remember that you can use our template to summarise and evaluate sources you’re thinking about using!

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It’s important to keep track of your sources with references to avoid plagiarism . It can be helpful to make an annotated bibliography, where you compile full reference information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

You can use our free APA Reference Generator for quick, correct, consistent citations.

To begin organising your literature review’s argument and structure, you need to understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly-visual platforms like Instagram and Snapchat – this is a gap that you could address in your own research.

There are various approaches to organising the body of a literature review. You should have a rough idea of your strategy before you start writing.

Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarising sources in order.

Try to analyse patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organise your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text, your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

If you are writing the literature review as part of your dissertation or thesis, reiterate your central problem or research question and give a brief summary of the scholarly context. You can emphasise the timeliness of the topic (“many recent studies have focused on the problem of x”) or highlight a gap in the literature (“while there has been much research on x, few researchers have taken y into consideration”).

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, make sure to follow these tips:

• Summarise and synthesise: give an overview of the main points of each source and combine them into a coherent whole.
• Analyse and interpret: don’t just paraphrase other researchers – add your own interpretations, discussing the significance of findings in relation to the literature as a whole.
• Critically evaluate: mention the strengths and weaknesses of your sources.
• Write in well-structured paragraphs: use transitions and topic sentences to draw connections, comparisons and contrasts.

In the conclusion, you should summarise the key findings you have taken from the literature and emphasise their significance.

If the literature review is part of your dissertation or thesis, reiterate how your research addresses gaps and contributes new knowledge, or discuss how you have drawn on existing theories and methods to build a framework for your research. This can lead directly into your methodology section.

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a dissertation , thesis, research paper , or proposal .

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarise yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your  dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

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• UConn Library
• Literature Review: The What, Why and How-to Guide
• Introduction

Literature Review: The What, Why and How-to Guide — Introduction

• Getting Started
• How to Pick a Topic
• Strategies to Find Sources
• Evaluating Sources & Lit. Reviews
• Tips for Writing Literature Reviews
• Writing Literature Review: Useful Sites
• Citation Resources
• Other Academic Writings

What are Literature Reviews?

So, what is a literature review? "A literature review is an account of what has been published on a topic by accredited scholars and researchers. In writing the literature review, your purpose is to convey to your reader what knowledge and ideas have been established on a topic, and what their strengths and weaknesses are. As a piece of writing, the literature review must be defined by a guiding concept (e.g., your research objective, the problem or issue you are discussing, or your argumentative thesis). It is not just a descriptive list of the material available, or a set of summaries." Taylor, D.  The literature review: A few tips on conducting it . University of Toronto Health Sciences Writing Centre.

Goals of Literature Reviews

What are the goals of creating a Literature Review?  A literature could be written to accomplish different aims:

• To develop a theory or evaluate an existing theory
• To summarize the historical or existing state of a research topic
• Identify a problem in a field of research 

Baumeister, R. F., & Leary, M. R. (1997). Writing narrative literature reviews .  Review of General Psychology , 1 (3), 311-320.

What kinds of sources require a Literature Review?

• A research paper assigned in a course
• A thesis or dissertation
• A grant proposal
• An article intended for publication in a journal

All these instances require you to collect what has been written about your research topic so that you can demonstrate how your own research sheds new light on the topic.

Types of Literature Reviews

What kinds of literature reviews are written?

Narrative review: The purpose of this type of review is to describe the current state of the research on a specific topic/research and to offer a critical analysis of the literature reviewed. Studies are grouped by research/theoretical categories, and themes and trends, strengths and weakness, and gaps are identified. The review ends with a conclusion section which summarizes the findings regarding the state of the research of the specific study, the gaps identify and if applicable, explains how the author's research will address gaps identify in the review and expand the knowledge on the topic reviewed.

• Example : Predictors and Outcomes of U.S. Quality Maternity Leave: A Review and Conceptual Framework:  10.1177/08948453211037398  

Systematic review : "The authors of a systematic review use a specific procedure to search the research literature, select the studies to include in their review, and critically evaluate the studies they find." (p. 139). Nelson, L. K. (2013). Research in Communication Sciences and Disorders . Plural Publishing.

• Example : The effect of leave policies on increasing fertility: a systematic review:  10.1057/s41599-022-01270-w

Meta-analysis : "Meta-analysis is a method of reviewing research findings in a quantitative fashion by transforming the data from individual studies into what is called an effect size and then pooling and analyzing this information. The basic goal in meta-analysis is to explain why different outcomes have occurred in different studies." (p. 197). Roberts, M. C., & Ilardi, S. S. (2003). Handbook of Research Methods in Clinical Psychology . Blackwell Publishing.

• Example : Employment Instability and Fertility in Europe: A Meta-Analysis:  10.1215/00703370-9164737

Meta-synthesis : "Qualitative meta-synthesis is a type of qualitative study that uses as data the findings from other qualitative studies linked by the same or related topic." (p.312). Zimmer, L. (2006). Qualitative meta-synthesis: A question of dialoguing with texts .  Journal of Advanced Nursing , 53 (3), 311-318.

• Example : Women’s perspectives on career successes and barriers: A qualitative meta-synthesis:  10.1177/05390184221113735

Literature Reviews in the Health Sciences

• UConn Health subject guide on systematic reviews Explanation of the different review types used in health sciences literature as well as tools to help you find the right review type
• << Previous: Getting Started
• Next: How to Pick a Topic >>
• Last Updated: Sep 21, 2022 2:16 PM
• URL: https://guides.lib.uconn.edu/literaturereview

What Is A Literature Review?

A plain-language explainer (with examples).

By:  Derek Jansen (MBA) & Kerryn Warren (PhD) | June 2020 (Updated May 2023)

If you’re faced with writing a dissertation or thesis, chances are you’ve encountered the term “literature review” . If you’re on this page, you’re probably not 100% what the literature review is all about. The good news is that you’ve come to the right place.

Literature Review 101

• What (exactly) is a literature review
• What’s the purpose of the literature review chapter
• How to find high-quality resources
• How to structure your literature review chapter
• Example of an actual literature review

What is a literature review?

The word “literature review” can refer to two related things that are part of the broader literature review process. The first is the task of  reviewing the literature  – i.e. sourcing and reading through the existing research relating to your research topic. The second is the  actual chapter  that you write up in your dissertation, thesis or research project. Let’s look at each of them:

Reviewing the literature

The first step of any literature review is to hunt down and  read through the existing research  that’s relevant to your research topic. To do this, you’ll use a combination of tools (we’ll discuss some of these later) to find journal articles, books, ebooks, research reports, dissertations, theses and any other credible sources of information that relate to your topic. You’ll then  summarise and catalogue these  for easy reference when you write up your literature review chapter. 

The literature review chapter

The second step of the literature review is to write the actual literature review chapter (this is usually the second chapter in a typical dissertation or thesis structure ). At the simplest level, the literature review chapter is an  overview of the key literature  that’s relevant to your research topic. This chapter should provide a smooth-flowing discussion of what research has already been done, what is known, what is unknown and what is contested in relation to your research topic. So, you can think of it as an  integrated review of the state of knowledge  around your research topic. 

What’s the purpose of a literature review?

The literature review chapter has a few important functions within your dissertation, thesis or research project. Let’s take a look at these:

Purpose #1 – Demonstrate your topic knowledge

The first function of the literature review chapter is, quite simply, to show the reader (or marker) that you  know what you’re talking about . In other words, a good literature review chapter demonstrates that you’ve read the relevant existing research and understand what’s going on – who’s said what, what’s agreed upon, disagreed upon and so on. This needs to be  more than just a summary  of who said what – it needs to integrate the existing research to  show how it all fits together  and what’s missing (which leads us to purpose #2, next). 

Purpose #2 – Reveal the research gap that you’ll fill

The second function of the literature review chapter is to  show what’s currently missing  from the existing research, to lay the foundation for your own research topic. In other words, your literature review chapter needs to show that there are currently “missing pieces” in terms of the bigger puzzle, and that  your study will fill one of those research gaps . By doing this, you are showing that your research topic is original and will help contribute to the body of knowledge. In other words, the literature review helps justify your research topic.  

Purpose #3 – Lay the foundation for your conceptual framework

The third function of the literature review is to form the  basis for a conceptual framework . Not every research topic will necessarily have a conceptual framework, but if your topic does require one, it needs to be rooted in your literature review. 

For example, let’s say your research aims to identify the drivers of a certain outcome – the factors which contribute to burnout in office workers. In this case, you’d likely develop a conceptual framework which details the potential factors (e.g. long hours, excessive stress, etc), as well as the outcome (burnout). Those factors would need to emerge from the literature review chapter – they can’t just come from your gut! 

So, in this case, the literature review chapter would uncover each of the potential factors (based on previous studies about burnout), which would then be modelled into a framework. 

Purpose #4 – To inform your methodology

The fourth function of the literature review is to  inform the choice of methodology  for your own research. As we’ve  discussed on the Grad Coach blog , your choice of methodology will be heavily influenced by your research aims, objectives and questions . Given that you’ll be reviewing studies covering a topic close to yours, it makes sense that you could learn a lot from their (well-considered) methodologies.

So, when you’re reviewing the literature, you’ll need to  pay close attention to the research design , methodology and methods used in similar studies, and use these to inform your methodology. Quite often, you’ll be able to  “borrow” from previous studies . This is especially true for quantitative studies , as you can use previously tried and tested measures and scales. 

How do I find articles for my literature review?

Finding quality journal articles is essential to crafting a rock-solid literature review. As you probably already know, not all research is created equally, and so you need to make sure that your literature review is  built on credible research . 

We could write an entire post on how to find quality literature (actually, we have ), but a good starting point is Google Scholar . Google Scholar is essentially the academic equivalent of Google, using Google’s powerful search capabilities to find relevant journal articles and reports. It certainly doesn’t cover every possible resource, but it’s a very useful way to get started on your literature review journey, as it will very quickly give you a good indication of what the  most popular pieces of research  are in your field.

One downside of Google Scholar is that it’s merely a search engine – that is, it lists the articles, but oftentimes  it doesn’t host the articles . So you’ll often hit a paywall when clicking through to journal websites. 

Thankfully, your university should provide you with access to their library, so you can find the article titles using Google Scholar and then search for them by name in your university’s online library. Your university may also provide you with access to  ResearchGate , which is another great source for existing research. 

Remember, the correct search keywords will be super important to get the right information from the start. So, pay close attention to the keywords used in the journal articles you read and use those keywords to search for more articles. If you can’t find a spoon in the kitchen, you haven’t looked in the right drawer. 

Need a helping hand?

How should I structure my literature review?

Unfortunately, there’s no generic universal answer for this one. The structure of your literature review will depend largely on your topic area and your research aims and objectives.

You could potentially structure your literature review chapter according to theme, group, variables , chronologically or per concepts in your field of research. We explain the main approaches to structuring your literature review here . You can also download a copy of our free literature review template to help you establish an initial structure.

In general, it’s also a good idea to start wide (i.e. the big-picture-level) and then narrow down, ending your literature review close to your research questions . However, there’s no universal one “right way” to structure your literature review. The most important thing is not to discuss your sources one after the other like a list – as we touched on earlier, your literature review needs to synthesise the research , not summarise it .

Ultimately, you need to craft your literature review so that it conveys the most important information effectively – it needs to tell a logical story in a digestible way. It’s no use starting off with highly technical terms and then only explaining what these terms mean later. Always assume your reader is not a subject matter expert and hold their hand through a journe y of the literature while keeping the functions of the literature review chapter (which we discussed earlier) front of mind.

Example of a literature review

In the video below, we walk you through a high-quality literature review from a dissertation that earned full distinction. This will give you a clearer view of what a strong literature review looks like in practice and hopefully provide some inspiration for your own. 

Wrapping Up

In this post, we’ve (hopefully) answered the question, “ what is a literature review? “. We’ve also considered the purpose and functions of the literature review, as well as how to find literature and how to structure the literature review chapter. If you’re keen to learn more, check out the literature review section of the Grad Coach blog , as well as our detailed video post covering how to write a literature review . 

Psst… there’s more!

This post is an extract from our bestselling short course, Literature Review Bootcamp . If you want to work smart, you don't want to miss this .

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What is a literature review? [with examples]

What is a literature review?

The purpose of a literature review, how to write a literature review, the format of a literature review, general formatting rules, the length of a literature review, literature review examples, frequently asked questions about literature reviews, related articles.

A literature review is an assessment of the sources in a chosen topic of research.

In a literature review, you’re expected to report on the existing scholarly conversation, without adding new contributions.

If you are currently writing one, you've come to the right place. In the following paragraphs, we will explain:

• the objective of a literature review
• how to write a literature review
• the basic format of a literature review

Tip: It’s not always mandatory to add a literature review in a paper. Theses and dissertations often include them, whereas research papers may not. Make sure to consult with your instructor for exact requirements.

The four main objectives of a literature review are:

• Studying the references of your research area
• Summarizing the main arguments
• Identifying current gaps, stances, and issues
• Presenting all of the above in a text

Ultimately, the main goal of a literature review is to provide the researcher with sufficient knowledge about the topic in question so that they can eventually make an intervention.

The format of a literature review is fairly standard. It includes an:

• introduction that briefly introduces the main topic
• body that includes the main discussion of the key arguments
• conclusion that highlights the gaps and issues of the literature

➡️ Take a look at our guide on how to write a literature review to learn more about how to structure a literature review.

First of all, a literature review should have its own labeled section. You should indicate clearly in the table of contents where the literature can be found, and you should label this section as “Literature Review.”

➡️ For more information on writing a thesis, visit our guide on how to structure a thesis .

There is no set amount of words for a literature review, so the length depends on the research. If you are working with a large amount of sources, it will be long. If your paper does not depend entirely on references, it will be short.

Take a look at these three theses featuring great literature reviews:

• School-Based Speech-Language Pathologist's Perceptions of Sensory Food Aversions in Children [ PDF , see page 20]
• Who's Writing What We Read: Authorship in Criminological Research [ PDF , see page 4]
• A Phenomenological Study of the Lived Experience of Online Instructors of Theological Reflection at Christian Institutions Accredited by the Association of Theological Schools [ PDF , see page 56]

Literature reviews are most commonly found in theses and dissertations. However, you find them in research papers as well.

There is no set amount of words for a literature review, so the length depends on the research. If you are working with a large amount of sources, then it will be long. If your paper does not depend entirely on references, then it will be short.

No. A literature review should have its own independent section. You should indicate clearly in the table of contents where the literature review can be found, and label this section as “Literature Review.”

The main goal of a literature review is to provide the researcher with sufficient knowledge about the topic in question so that they can eventually make an intervention.

• University of Oregon Libraries
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How to Write a Literature Review

What's a literature review.

• Literature Reviews: A Recap
• Reading Journal Articles
• Does it Describe a Literature Review?
• 1. Identify the Question
• 2. Review Discipline Styles
• Searching Article Databases
• Finding Full-Text of an Article
• Citation Chaining
• When to Stop Searching
• 4. Manage Your References
• 5. Critically Analyze and Evaluate
• 6. Synthesize
• 7. Write a Literature Review

What's a Literature Review? 

A literature review (or "lit review," for short) is an in-depth critical analysis of published scholarly research related to a specific topic. Published scholarly research (aka, "the literature") may include journal articles, books, book chapters, dissertations and thesis, or conference proceedings. 

A solid lit review must:

• be organized around and related directly to the thesis or research question you're developing
• synthesize results into a summary of what is and is not known
• identify areas of controversy in the literature
• formulate questions that need further research

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Introduction to Literature Reviews

Introduction.

• Step One: Define
• Step Two: Research
• Step Three: Write
• Suggested Readings

A literature review is a written work that :

• Compiles significant research published on a topic by accredited scholars and researchers;
• —Surveys scholarly articles, books, dissertations, conference proceedings, and other sources;
• —Examines contrasting perspectives, theoretical approaches, methodologies, findings, results, conclusions.
• —Reviews critically, analyzes, and synthesizes existing research on a topic; and,
• Performs a thorough “re” view, “overview”, or “look again” of past and current works on a subject, issue, or theory.

From these analyses, the writer then offers an overview of the current status of a particular area of knowledge from both a practical and theoretical perspective.

Literature reviews are important because they are usually a  required  step in a thesis proposal (Master's or PhD). The proposal will not be well-supported without a literature review. Also, literature reviews are important because they help you learn important authors and ideas in your field. This is useful for your coursework and your writing. Knowing key authors also helps you become acquainted with other researchers in your field.

Look at this diagram and imagine that your research is the "something new." This shows how your research should relate to major works and other sources.

Olivia Whitfield | Graduate Reference Assistant | 2012-2015

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• Last Updated: Jun 28, 2023 5:49 PM
• URL: https://libraryguides.missouri.edu/literaturereview

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Organizing Your Social Sciences Research Paper

• 5. The Literature Review
• Purpose of Guide
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A literature review surveys prior research published in books, scholarly articles, and any other sources relevant to a particular issue, area of research, or theory, and by so doing, provides a description, summary, and critical evaluation of these works in relation to the research problem being investigated. Literature reviews are designed to provide an overview of sources you have used in researching a particular topic and to demonstrate to your readers how your research fits within existing scholarship about the topic.

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . Fourth edition. Thousand Oaks, CA: SAGE, 2014.

Importance of a Good Literature Review

A literature review may consist of simply a summary of key sources, but in the social sciences, a literature review usually has an organizational pattern and combines both summary and synthesis, often within specific conceptual categories . A summary is a recap of the important information of the source, but a synthesis is a re-organization, or a reshuffling, of that information in a way that informs how you are planning to investigate a research problem. The analytical features of a literature review might:

• Give a new interpretation of old material or combine new with old interpretations,
• Trace the intellectual progression of the field, including major debates,
• Depending on the situation, evaluate the sources and advise the reader on the most pertinent or relevant research, or
• Usually in the conclusion of a literature review, identify where gaps exist in how a problem has been researched to date.

Given this, the purpose of a literature review is to:

• Place each work in the context of its contribution to understanding the research problem being studied.
• Describe the relationship of each work to the others under consideration.
• Identify new ways to interpret prior research.
• Reveal any gaps that exist in the literature.
• Resolve conflicts amongst seemingly contradictory previous studies.
• Identify areas of prior scholarship to prevent duplication of effort.
• Point the way in fulfilling a need for additional research.
• Locate your own research within the context of existing literature [very important].

Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper. 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . Los Angeles, CA: SAGE, 2011; Knopf, Jeffrey W. "Doing a Literature Review." PS: Political Science and Politics 39 (January 2006): 127-132; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012.

Types of Literature Reviews

It is important to think of knowledge in a given field as consisting of three layers. First, there are the primary studies that researchers conduct and publish. Second are the reviews of those studies that summarize and offer new interpretations built from and often extending beyond the primary studies. Third, there are the perceptions, conclusions, opinion, and interpretations that are shared informally among scholars that become part of the body of epistemological traditions within the field.

In composing a literature review, it is important to note that it is often this third layer of knowledge that is cited as "true" even though it often has only a loose relationship to the primary studies and secondary literature reviews. Given this, while literature reviews are designed to provide an overview and synthesis of pertinent sources you have explored, there are a number of approaches you could adopt depending upon the type of analysis underpinning your study.

Argumentative Review This form examines literature selectively in order to support or refute an argument, deeply embedded assumption, or philosophical problem already established in the literature. The purpose is to develop a body of literature that establishes a contrarian viewpoint. Given the value-laden nature of some social science research [e.g., educational reform; immigration control], argumentative approaches to analyzing the literature can be a legitimate and important form of discourse. However, note that they can also introduce problems of bias when they are used to make summary claims of the sort found in systematic reviews [see below].

Integrative Review Considered a form of research that reviews, critiques, and synthesizes representative literature on a topic in an integrated way such that new frameworks and perspectives on the topic are generated. The body of literature includes all studies that address related or identical hypotheses or research problems. A well-done integrative review meets the same standards as primary research in regard to clarity, rigor, and replication. This is the most common form of review in the social sciences.

Historical Review Few things rest in isolation from historical precedent. Historical literature reviews focus on examining research throughout a period of time, often starting with the first time an issue, concept, theory, phenomena emerged in the literature, then tracing its evolution within the scholarship of a discipline. The purpose is to place research in a historical context to show familiarity with state-of-the-art developments and to identify the likely directions for future research.

Methodological Review A review does not always focus on what someone said [findings], but how they came about saying what they say [method of analysis]. Reviewing methods of analysis provides a framework of understanding at different levels [i.e. those of theory, substantive fields, research approaches, and data collection and analysis techniques], how researchers draw upon a wide variety of knowledge ranging from the conceptual level to practical documents for use in fieldwork in the areas of ontological and epistemological consideration, quantitative and qualitative integration, sampling, interviewing, data collection, and data analysis. This approach helps highlight ethical issues which you should be aware of and consider as you go through your own study.

Systematic Review This form consists of an overview of existing evidence pertinent to a clearly formulated research question, which uses pre-specified and standardized methods to identify and critically appraise relevant research, and to collect, report, and analyze data from the studies that are included in the review. The goal is to deliberately document, critically evaluate, and summarize scientifically all of the research about a clearly defined research problem . Typically it focuses on a very specific empirical question, often posed in a cause-and-effect form, such as "To what extent does A contribute to B?" This type of literature review is primarily applied to examining prior research studies in clinical medicine and allied health fields, but it is increasingly being used in the social sciences.

Theoretical Review The purpose of this form is to examine the corpus of theory that has accumulated in regard to an issue, concept, theory, phenomena. The theoretical literature review helps to establish what theories already exist, the relationships between them, to what degree the existing theories have been investigated, and to develop new hypotheses to be tested. Often this form is used to help establish a lack of appropriate theories or reveal that current theories are inadequate for explaining new or emerging research problems. The unit of analysis can focus on a theoretical concept or a whole theory or framework.

NOTE: Most often the literature review will incorporate some combination of types. For example, a review that examines literature supporting or refuting an argument, assumption, or philosophical problem related to the research problem will also need to include writing supported by sources that establish the history of these arguments in the literature.

Baumeister, Roy F. and Mark R. Leary. "Writing Narrative Literature Reviews."  Review of General Psychology 1 (September 1997): 311-320; Mark R. Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Kennedy, Mary M. "Defining a Literature." Educational Researcher 36 (April 2007): 139-147; Petticrew, Mark and Helen Roberts. Systematic Reviews in the Social Sciences: A Practical Guide . Malden, MA: Blackwell Publishers, 2006; Torracro, Richard. "Writing Integrative Literature Reviews: Guidelines and Examples." Human Resource Development Review 4 (September 2005): 356-367; Rocco, Tonette S. and Maria S. Plakhotnik. "Literature Reviews, Conceptual Frameworks, and Theoretical Frameworks: Terms, Functions, and Distinctions." Human Ressource Development Review 8 (March 2008): 120-130; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

Structure and Writing Style

I.  Thinking About Your Literature Review

The structure of a literature review should include the following in support of understanding the research problem :

• An overview of the subject, issue, or theory under consideration, along with the objectives of the literature review,
• Division of works under review into themes or categories [e.g. works that support a particular position, those against, and those offering alternative approaches entirely],
• An explanation of how each work is similar to and how it varies from the others,
• Conclusions as to which pieces are best considered in their argument, are most convincing of their opinions, and make the greatest contribution to the understanding and development of their area of research.

The critical evaluation of each work should consider :

• Provenance -- what are the author's credentials? Are the author's arguments supported by evidence [e.g. primary historical material, case studies, narratives, statistics, recent scientific findings]?
• Methodology -- were the techniques used to identify, gather, and analyze the data appropriate to addressing the research problem? Was the sample size appropriate? Were the results effectively interpreted and reported?
• Objectivity -- is the author's perspective even-handed or prejudicial? Is contrary data considered or is certain pertinent information ignored to prove the author's point?
• Persuasiveness -- which of the author's theses are most convincing or least convincing?
• Validity -- are the author's arguments and conclusions convincing? Does the work ultimately contribute in any significant way to an understanding of the subject?

II.  Development of the Literature Review

Four Basic Stages of Writing 1.  Problem formulation -- which topic or field is being examined and what are its component issues? 2.  Literature search -- finding materials relevant to the subject being explored. 3.  Data evaluation -- determining which literature makes a significant contribution to the understanding of the topic. 4.  Analysis and interpretation -- discussing the findings and conclusions of pertinent literature.

Consider the following issues before writing the literature review: Clarify If your assignment is not specific about what form your literature review should take, seek clarification from your professor by asking these questions: 1.  Roughly how many sources would be appropriate to include? 2.  What types of sources should I review (books, journal articles, websites; scholarly versus popular sources)? 3.  Should I summarize, synthesize, or critique sources by discussing a common theme or issue? 4.  Should I evaluate the sources in any way beyond evaluating how they relate to understanding the research problem? 5.  Should I provide subheadings and other background information, such as definitions and/or a history? Find Models Use the exercise of reviewing the literature to examine how authors in your discipline or area of interest have composed their literature review sections. Read them to get a sense of the types of themes you might want to look for in your own research or to identify ways to organize your final review. The bibliography or reference section of sources you've already read, such as required readings in the course syllabus, are also excellent entry points into your own research. Narrow the Topic The narrower your topic, the easier it will be to limit the number of sources you need to read in order to obtain a good survey of relevant resources. Your professor will probably not expect you to read everything that's available about the topic, but you'll make the act of reviewing easier if you first limit scope of the research problem. A good strategy is to begin by searching the USC Libraries Catalog for recent books about the topic and review the table of contents for chapters that focuses on specific issues. You can also review the indexes of books to find references to specific issues that can serve as the focus of your research. For example, a book surveying the history of the Israeli-Palestinian conflict may include a chapter on the role Egypt has played in mediating the conflict, or look in the index for the pages where Egypt is mentioned in the text. Consider Whether Your Sources are Current Some disciplines require that you use information that is as current as possible. This is particularly true in disciplines in medicine and the sciences where research conducted becomes obsolete very quickly as new discoveries are made. However, when writing a review in the social sciences, a survey of the history of the literature may be required. In other words, a complete understanding the research problem requires you to deliberately examine how knowledge and perspectives have changed over time. Sort through other current bibliographies or literature reviews in the field to get a sense of what your discipline expects. You can also use this method to explore what is considered by scholars to be a "hot topic" and what is not.

III.  Ways to Organize Your Literature Review

Chronology of Events If your review follows the chronological method, you could write about the materials according to when they were published. This approach should only be followed if a clear path of research building on previous research can be identified and that these trends follow a clear chronological order of development. For example, a literature review that focuses on continuing research about the emergence of German economic power after the fall of the Soviet Union. By Publication Order your sources by publication chronology, then, only if the order demonstrates a more important trend. For instance, you could order a review of literature on environmental studies of brown fields if the progression revealed, for example, a change in the soil collection practices of the researchers who wrote and/or conducted the studies. Thematic [“conceptual categories”] A thematic literature review is the most common approach to summarizing prior research in the social and behavioral sciences. Thematic reviews are organized around a topic or issue, rather than the progression of time, although the progression of time may still be incorporated into a thematic review. For example, a review of the Internet’s impact on American presidential politics could focus on the development of online political satire. While the study focuses on one topic, the Internet’s impact on American presidential politics, it would still be organized chronologically reflecting technological developments in media. The difference in this example between a "chronological" and a "thematic" approach is what is emphasized the most: themes related to the role of the Internet in presidential politics. Note that more authentic thematic reviews tend to break away from chronological order. A review organized in this manner would shift between time periods within each section according to the point being made. Methodological A methodological approach focuses on the methods utilized by the researcher. For the Internet in American presidential politics project, one methodological approach would be to look at cultural differences between the portrayal of American presidents on American, British, and French websites. Or the review might focus on the fundraising impact of the Internet on a particular political party. A methodological scope will influence either the types of documents in the review or the way in which these documents are discussed.

Other Sections of Your Literature Review Once you've decided on the organizational method for your literature review, the sections you need to include in the paper should be easy to figure out because they arise from your organizational strategy. In other words, a chronological review would have subsections for each vital time period; a thematic review would have subtopics based upon factors that relate to the theme or issue. However, sometimes you may need to add additional sections that are necessary for your study, but do not fit in the organizational strategy of the body. What other sections you include in the body is up to you. However, only include what is necessary for the reader to locate your study within the larger scholarship about the research problem.

Here are examples of other sections, usually in the form of a single paragraph, you may need to include depending on the type of review you write:

• Current Situation : Information necessary to understand the current topic or focus of the literature review.
• Sources Used : Describes the methods and resources [e.g., databases] you used to identify the literature you reviewed.
• History : The chronological progression of the field, the research literature, or an idea that is necessary to understand the literature review, if the body of the literature review is not already a chronology.
• Selection Methods : Criteria you used to select (and perhaps exclude) sources in your literature review. For instance, you might explain that your review includes only peer-reviewed [i.e., scholarly] sources.
• Standards : Description of the way in which you present your information.
• Questions for Further Research : What questions about the field has the review sparked? How will you further your research as a result of the review?

IV.  Writing Your Literature Review

Once you've settled on how to organize your literature review, you're ready to write each section. When writing your review, keep in mind these issues.

Use Evidence A literature review section is, in this sense, just like any other academic research paper. Your interpretation of the available sources must be backed up with evidence [citations] that demonstrates that what you are saying is valid. Be Selective Select only the most important points in each source to highlight in the review. The type of information you choose to mention should relate directly to the research problem, whether it is thematic, methodological, or chronological. Related items that provide additional information, but that are not key to understanding the research problem, can be included in a list of further readings . Use Quotes Sparingly Some short quotes are appropriate if you want to emphasize a point, or if what an author stated cannot be easily paraphrased. Sometimes you may need to quote certain terminology that was coined by the author, is not common knowledge, or taken directly from the study. Do not use extensive quotes as a substitute for using your own words in reviewing the literature. Summarize and Synthesize Remember to summarize and synthesize your sources within each thematic paragraph as well as throughout the review. Recapitulate important features of a research study, but then synthesize it by rephrasing the study's significance and relating it to your own work and the work of others. Keep Your Own Voice While the literature review presents others' ideas, your voice [the writer's] should remain front and center. For example, weave references to other sources into what you are writing but maintain your own voice by starting and ending the paragraph with your own ideas and wording. Use Caution When Paraphrasing When paraphrasing a source that is not your own, be sure to represent the author's information or opinions accurately and in your own words. Even when paraphrasing an author’s work, you still must provide a citation to that work.

V.  Common Mistakes to Avoid

These are the most common mistakes made in reviewing social science research literature.

• Sources in your literature review do not clearly relate to the research problem;
• You do not take sufficient time to define and identify the most relevant sources to use in the literature review related to the research problem;
• Relies exclusively on secondary analytical sources rather than including relevant primary research studies or data;
• Uncritically accepts another researcher's findings and interpretations as valid, rather than examining critically all aspects of the research design and analysis;
• Does not describe the search procedures that were used in identifying the literature to review;
• Reports isolated statistical results rather than synthesizing them in chi-squared or meta-analytic methods; and,
• Only includes research that validates assumptions and does not consider contrary findings and alternative interpretations found in the literature.

Cook, Kathleen E. and Elise Murowchick. “Do Literature Review Skills Transfer from One Course to Another?” Psychology Learning and Teaching 13 (March 2014): 3-11; Fink, Arlene. Conducting Research Literature Reviews: From the Internet to Paper . 2nd ed. Thousand Oaks, CA: Sage, 2005; Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1998; Jesson, Jill. Doing Your Literature Review: Traditional and Systematic Techniques . London: SAGE, 2011; Literature Review Handout. Online Writing Center. Liberty University; Literature Reviews. The Writing Center. University of North Carolina; Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: SAGE, 2016; Ridley, Diana. The Literature Review: A Step-by-Step Guide for Students . 2nd ed. Los Angeles, CA: SAGE, 2012; Randolph, Justus J. “A Guide to Writing the Dissertation Literature Review." Practical Assessment, Research, and Evaluation. vol. 14, June 2009; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016; Taylor, Dena. The Literature Review: A Few Tips On Conducting It. University College Writing Centre. University of Toronto; Writing a Literature Review. Academic Skills Centre. University of Canberra.

Writing Tip

Break Out of Your Disciplinary Box!

Thinking interdisciplinarily about a research problem can be a rewarding exercise in applying new ideas, theories, or concepts to an old problem. For example, what might cultural anthropologists say about the continuing conflict in the Middle East? In what ways might geographers view the need for better distribution of social service agencies in large cities than how social workers might study the issue? You don’t want to substitute a thorough review of core research literature in your discipline for studies conducted in other fields of study. However, particularly in the social sciences, thinking about research problems from multiple vectors is a key strategy for finding new solutions to a problem or gaining a new perspective. Consult with a librarian about identifying research databases in other disciplines; almost every field of study has at least one comprehensive database devoted to indexing its research literature.

Frodeman, Robert. The Oxford Handbook of Interdisciplinarity . New York: Oxford University Press, 2010.

Another Writing Tip

Don't Just Review for Content!

While conducting a review of the literature, maximize the time you devote to writing this part of your paper by thinking broadly about what you should be looking for and evaluating. Review not just what scholars are saying, but how are they saying it. Some questions to ask:

• How are they organizing their ideas?
• What methods have they used to study the problem?
• What theories have been used to explain, predict, or understand their research problem?
• What sources have they cited to support their conclusions?
• How have they used non-textual elements [e.g., charts, graphs, figures, etc.] to illustrate key points?

When you begin to write your literature review section, you'll be glad you dug deeper into how the research was designed and constructed because it establishes a means for developing more substantial analysis and interpretation of the research problem.

Hart, Chris. Doing a Literature Review: Releasing the Social Science Research Imagination . Thousand Oaks, CA: Sage Publications, 1 998.

Yet Another Writing Tip

When Do I Know I Can Stop Looking and Move On?

Here are several strategies you can utilize to assess whether you've thoroughly reviewed the literature:

• Look for repeating patterns in the research findings . If the same thing is being said, just by different people, then this likely demonstrates that the research problem has hit a conceptual dead end. At this point consider: Does your study extend current research?  Does it forge a new path? Or, does is merely add more of the same thing being said?
• Look at sources the authors cite to in their work . If you begin to see the same researchers cited again and again, then this is often an indication that no new ideas have been generated to address the research problem.
• Search Google Scholar to identify who has subsequently cited leading scholars already identified in your literature review [see next sub-tab]. This is called citation tracking and there are a number of sources that can help you identify who has cited whom, particularly scholars from outside of your discipline. Here again, if the same authors are being cited again and again, this may indicate no new literature has been written on the topic.

Onwuegbuzie, Anthony J. and Rebecca Frels. Seven Steps to a Comprehensive Literature Review: A Multimodal and Cultural Approach . Los Angeles, CA: Sage, 2016; Sutton, Anthea. Systematic Approaches to a Successful Literature Review . Los Angeles, CA: Sage Publications, 2016.

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What is a Literature Review?

So, what is a literature review .

"A literature review is an account of what has been published on a topic by accredited scholars and researchers. In writing the literature review, your purpose is to convey to your reader what knowledge and ideas have been established on a topic, and what their strengths and weaknesses are. As a piece of writing, the literature review must be defined by a guiding concept (e.g., your research objective, the problem or issue you are discussing, or your argumentative thesis). It is not just a descriptive list of the material available or a set of summaries." - Quote from Taylor, D. (n.d)."The Literature Review: A Few Tips on Conducting it".

• Citation: "The Literature Review: A Few Tips on Conducting it"

What kinds of literature reviews are written?

Each field has a particular way to do reviews for academic research literature. In the social sciences and humanities the most common are:

• Narrative Reviews: The purpose of this type of review is to describe the current state of the research on a specific research topic and to offer a critical analysis of the literature reviewed. Studies are grouped by research/theoretical categories, and themes and trends, strengths and weaknesses, and gaps are identified. The review ends with a conclusion section that summarizes the findings regarding the state of the research of the specific study, the gaps identify and if applicable, explains how the author's research will address gaps identify in the review and expand the knowledge on the topic reviewed.
• Book review essays/ Historiographical review essays : A type of literature review typical in History and related fields, e.g., Latin American studies. For example, the Latin American Research Review explains that the purpose of this type of review is to “(1) to familiarize readers with the subject, approach, arguments, and conclusions found in a group of books whose common focus is a historical period; a country or region within Latin America; or a practice, development, or issue of interest to specialists and others; (2) to locate these books within current scholarship, critical methodologies, and approaches; and (3) to probe the relation of these new books to previous work on the subject, especially canonical texts. Unlike individual book reviews, the cluster reviews found in LARR seek to address the state of the field or discipline and not solely the works at issue.” - LARR

What are the Goals of Creating a Literature Review?

• To develop a theory or evaluate an existing theory
• To summarize the historical or existing state of a research topic
• Identify a problem in a field of research 
• Baumeister, R.F. & Leary, M.R. (1997). "Writing narrative literature reviews," Review of General Psychology , 1(3), 311-320.

When do you need to write a Literature Review?

• When writing a prospectus or a thesis/dissertation
• When writing a research paper
• When writing a grant proposal

In all these cases you need to dedicate a chapter in these works to showcase what has been written about your research topic and to point out how your own research will shed new light into a body of scholarship.

Where I can find examples of Literature Reviews?

Note:  In the humanities, even if they don't use the term "literature review", they may have a dedicated  chapter that reviewed the "critical bibliography" or they incorporated that review in the introduction or first chapter of the dissertation, book, or article.

• UCSB electronic theses and dissertations In partnership with the Graduate Division, the UC Santa Barbara Library is making available theses and dissertations produced by UCSB students. Currently included in ADRL are theses and dissertations that were originally filed electronically, starting in 2011. In future phases of ADRL, all theses and dissertations created by UCSB students may be digitized and made available.

Where to Find Standalone Literature Reviews

Literature reviews are also written as standalone articles as a way to survey a particular research topic in-depth. This type of literature review looks at a topic from a historical perspective to see how the understanding of the topic has changed over time. 

• Find e-Journals for Standalone Literature Reviews The best way to get familiar with and to learn how to write literature reviews is by reading them. You can use our Journal Search option to find journals that specialize in publishing literature reviews from major disciplines like anthropology, sociology, etc. Usually these titles are called, "Annual Review of [discipline name] OR [Discipline name] Review. This option works best if you know the title of the publication you are looking for. Below are some examples of these journals! more... less... Journal Search can be found by hovering over the link for Research on the library website.

Social Sciences

• Annual Review of Anthropology
• Annual Review of Political Science
• Annual Review of Sociology
• Ethnic Studies Review

Hard science and health sciences:

• Annual Review of Biomedical Data Science
• Annual Review of Materials Science
• Systematic Review From journal site: "The journal Systematic Reviews encompasses all aspects of the design, conduct, and reporting of systematic reviews" in the health sciences.
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What is a literature review?

A literature review is an integrated analysis -- not just a summary-- of scholarly writings and other relevant evidence related directly to your research question.  That is, it represents a synthesis of the evidence that provides background information on your topic and shows a association between the evidence and your research question.

A literature review may be a stand alone work or the introduction to a larger research paper, depending on the assignment.  Rely heavily on the guidelines your instructor has given you.

Why is it important?

A literature review is important because it:

• Explains the background of research on a topic.
• Demonstrates why a topic is significant to a subject area.
• Discovers relationships between research studies/ideas.
• Identifies major themes, concepts, and researchers on a topic.
• Identifies critical gaps and points of disagreement.
• Discusses further research questions that logically come out of the previous studies.

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1. Choose a topic. Define your research question.

Your literature review should be guided by your central research question.  The literature represents background and research developments related to a specific research question, interpreted and analyzed by you in a synthesized way.

• Make sure your research question is not too broad or too narrow.  Is it manageable?
• Begin writing down terms that are related to your question. These will be useful for searches later.
• If you have the opportunity, discuss your topic with your professor and your class mates.

2. Decide on the scope of your review

How many studies do you need to look at? How comprehensive should it be? How many years should it cover? 

• This may depend on your assignment.  How many sources does the assignment require?

3. Select the databases you will use to conduct your searches.

Make a list of the databases you will search. 

Where to find databases:

• use the tabs on this guide
• Find other databases in the Nursing Information Resources web page
• More on the Medical Library web page
• ... and more on the Yale University Library web page

4. Conduct your searches to find the evidence. Keep track of your searches.

• Use the key words in your question, as well as synonyms for those words, as terms in your search. Use the database tutorials for help.
• Save the searches in the databases. This saves time when you want to redo, or modify, the searches. It is also helpful to use as a guide is the searches are not finding any useful results.
• Review the abstracts of research studies carefully. This will save you time.
• Use the bibliographies and references of research studies you find to locate others.
• Check with your professor, or a subject expert in the field, if you are missing any key works in the field.
• Ask your librarian for help at any time.
• Use a citation manager, such as EndNote as the repository for your citations. See the EndNote tutorials for help.

Review the literature

Some questions to help you analyze the research:

• What was the research question of the study you are reviewing? What were the authors trying to discover?
• Was the research funded by a source that could influence the findings?
• What were the research methodologies? Analyze its literature review, the samples and variables used, the results, and the conclusions.
• Does the research seem to be complete? Could it have been conducted more soundly? What further questions does it raise?
• If there are conflicting studies, why do you think that is?
• How are the authors viewed in the field? Has this study been cited? If so, how has it been analyzed?

Tips: 

• Review the abstracts carefully.  
• Keep careful notes so that you may track your thought processes during the research process.
• Create a matrix of the studies for easy analysis, and synthesis, across all of the studies.
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Literature review

A general guide on how to conduct and write a literature review.

Please check course or programme information and materials provided by teaching staff, including your project supervisor, for subject-specific guidance.

What is a literature review?

A literature review is a piece of academic writing demonstrating knowledge and understanding of the academic literature on a specific topic placed in context.  A literature review also includes a critical evaluation of the material; this is why it is called a literature review rather than a literature report. It is a process of reviewing the literature, as well as a form of writing.

To illustrate the difference between reporting and reviewing, think about television or film review articles.  These articles include content such as a brief synopsis or the key points of the film or programme plus the critic’s own evaluation.  Similarly the two main objectives of a literature review are firstly the content covering existing research, theories and evidence, and secondly your own critical evaluation and discussion of this content. 

Usually a literature review forms a section or part of a dissertation, research project or long essay.  However, it can also be set and assessed as a standalone piece of work.

What is the purpose of a literature review?

…your task is to build an argument, not a library. Rudestam, K.E. and Newton, R.R. (1992) Surviving your dissertation: A comprehensive guide to content and process. California: Sage, p49.

In a larger piece of written work, such as a dissertation or project, a literature review is usually one of the first tasks carried out after deciding on a topic.  Reading combined with critical analysis can help to refine a topic and frame research questions.  Conducting a literature review establishes your familiarity with and understanding of current research in a particular field before carrying out a new investigation. After doing a literature review, you should know what research has already been done and be able to identify what is unknown within your topic.

When doing and writing a literature review, it is good practice to:

• summarise and analyse previous research and theories;
• identify areas of controversy and contested claims;
• highlight any gaps that may exist in research to date.

Conducting a literature review

Focusing on different aspects of your literature review can be useful to help plan, develop, refine and write it.  You can use and adapt the prompt questions in our worksheet below at different points in the process of researching and writing your review.  These are suggestions to get you thinking and writing.

Developing and refining your literature review (pdf)

Developing and refining your literature review (Word)

Developing and refining your literature review (Word rtf)

Writing a literature review has a lot in common with other assignment tasks.  There is advice on our other pages about thinking critically, reading strategies and academic writing.  Our literature review top tips suggest some specific things you can do to help you submit a successful review.

Literature review top tips (pdf)

Literature review top tips (Word rtf)

Our reading page includes strategies and advice on using books and articles and a notes record sheet grid you can use.

Reading at university

The Academic writing page suggests ways to organise and structure information from a range of sources and how you can develop your argument as you read and write.

Academic writing

The Critical thinking page has advice on how to be a more critical researcher and a form you can use to help you think and break down the stages of developing your argument.

Critical thinking

As with other forms of academic writing, your literature review needs to demonstrate good academic practice by following the Code of Student Conduct and acknowledging the work of others through citing and referencing your sources.  

Good academic practice

As with any writing task, you will need to review, edit and rewrite sections of your literature review.  The Editing and proofreading page includes tips on how to do this and strategies for standing back and thinking about your structure and checking the flow of your argument.

Editing and proofreading

Guidance on literature searching from the University Library

The Academic Support Librarians have developed LibSmart I and II, Learn courses to help you develop and enhance your digital research skills and capabilities; from getting started with the Library to managing data for your dissertation.

Searching using the library’s DiscoverEd tool: DiscoverEd

Finding resources in your subject: Subject guides

The Academic Support Librarians also provide one-to-one appointments to help you develop your research strategies.

1 to 1 support for literature searching and systematic reviews

Advice to help you optimise use of Google Scholar, Google Books and Google for your research and study: Using Google

Managing and curating your references

A referencing management tool can help you to collect and organise and your source material to produce a bibliography or reference list. 

Referencing and reference management

Information Services provide access to Cite them right online which is a guide to the main referencing systems and tells you how to reference just about any source (EASE log-in may be required).

Cite them right

Published study guides

There are a number of scholarship skills books and guides available which can help with writing a literature review.  Our Resource List of study skills guides includes sections on Referencing, Dissertation and project writing and Literature reviews.

Study skills guides

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What is a literature review?

A literature review is a critical analysis of the literature related to your research topic. It evaluates and critiques the literature to establish a theoretical framework for your research topic and/or identify a gap in the existing research that your research will address.

A literature review is not a summary of the literature. You need to engage deeply and critically with the literature. Your literature review should show your understanding of the literature related to your research topic and lead to presenting a rationale for your research.

A literature review focuses on:

• the context of the topic
• key concepts, ideas, theories and methodologies
• key researchers, texts and seminal works
• major issues and debates
• identifying conflicting evidence
• the main questions that have been asked around the topic
• the organisation of knowledge on the topic
• definitions, particularly those that are contested
• showing how your research will advance scholarly knowledge (generally referred to as identifying the ‘gap’).

This module will guide you through the functions of a literature review; the typical process of conducting a literature review (including searching for literature and taking notes); structuring your literature review within your thesis and organising its internal ideas; and styling the language of your literature review.

The purposes of a literature review

A literature review serves two main purposes:

1) To show awareness of the present state of knowledge in a particular field, including:

• seminal authors
• the main empirical research
• theoretical positions
• controversies
• breakthroughs as well as links to other related areas of knowledge.

2) To provide a foundation for the author’s research. To do that, the literature review needs to:

• help the researcher define a hypothesis or a research question, and how answering the question will contribute to the body of knowledge;
• provide a rationale for investigating the problem and the selected methodology;
• provide a particular theoretical lens, support the argument, or identify gaps.

Before you engage further with this module, try the quiz below to see how much you already know about literature reviews.

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Literature review.

• What is a Literature Review?
• What is Its Purpose?
• 1. Select a Topic
• 2. Set the Topic in Context
• 3. Types of Information Sources
• 4. Use Information Sources
• 5. Get the Information
• 6. Organize / Manage the Information
• 7. Position the Literature Review
• 8. Write the Literature Review

A literature review is a comprehensive summary of previous research on a topic. The literature review surveys scholarly articles, books, and other sources relevant to a particular area of research.  The review should enumerate, describe, summarize, objectively evaluate and clarify this previous research.  It should give a theoretical base for the research and help you (the author) determine the nature of your research.  The literature review acknowledges the work of previous researchers, and in so doing, assures the reader that your work has been well conceived.  It is assumed that by mentioning a previous work in the field of study, that the author has read, evaluated, and assimiliated that work into the work at hand.

A literature review creates a "landscape" for the reader, giving her or him a full understanding of the developments in the field.  This landscape informs the reader that the author has indeed assimilated all (or the vast majority of) previous, significant works in the field into her or his research. 

 "In writing the literature review, the purpose is to convey to the reader what knowledge and ideas have been established on a topic, and what their strengths and weaknesses are. The literature review must be defined by a guiding concept (eg. your research objective, the problem or issue you are discussing, or your argumentative thesis). It is not just a descriptive list of the material available, or a set of summaries.( http://www.writing.utoronto.ca/advice/specific-types-of-writing/literature-review )

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Literature Review – Types Writing Guide and Examples

Table of Contents

Literature Review

Definition:

A literature review is a comprehensive and critical analysis of the existing literature on a particular topic or research question. It involves identifying, evaluating, and synthesizing relevant literature, including scholarly articles, books, and other sources, to provide a summary and critical assessment of what is known about the topic.

Types of Literature Review

Types of Literature Review are as follows:

• Narrative literature review : This type of review involves a comprehensive summary and critical analysis of the available literature on a particular topic or research question. It is often used as an introductory section of a research paper.
• Systematic literature review: This is a rigorous and structured review that follows a pre-defined protocol to identify, evaluate, and synthesize all relevant studies on a specific research question. It is often used in evidence-based practice and systematic reviews.
• Meta-analysis: This is a quantitative review that uses statistical methods to combine data from multiple studies to derive a summary effect size. It provides a more precise estimate of the overall effect than any individual study.
• Scoping review: This is a preliminary review that aims to map the existing literature on a broad topic area to identify research gaps and areas for further investigation.
• Critical literature review : This type of review evaluates the strengths and weaknesses of the existing literature on a particular topic or research question. It aims to provide a critical analysis of the literature and identify areas where further research is needed.
• Conceptual literature review: This review synthesizes and integrates theories and concepts from multiple sources to provide a new perspective on a particular topic. It aims to provide a theoretical framework for understanding a particular research question.
• Rapid literature review: This is a quick review that provides a snapshot of the current state of knowledge on a specific research question or topic. It is often used when time and resources are limited.
• Thematic literature review : This review identifies and analyzes common themes and patterns across a body of literature on a particular topic. It aims to provide a comprehensive overview of the literature and identify key themes and concepts.
• Realist literature review: This review is often used in social science research and aims to identify how and why certain interventions work in certain contexts. It takes into account the context and complexities of real-world situations.
• State-of-the-art literature review : This type of review provides an overview of the current state of knowledge in a particular field, highlighting the most recent and relevant research. It is often used in fields where knowledge is rapidly evolving, such as technology or medicine.
• Integrative literature review: This type of review synthesizes and integrates findings from multiple studies on a particular topic to identify patterns, themes, and gaps in the literature. It aims to provide a comprehensive understanding of the current state of knowledge on a particular topic.
• Umbrella literature review : This review is used to provide a broad overview of a large and diverse body of literature on a particular topic. It aims to identify common themes and patterns across different areas of research.
• Historical literature review: This type of review examines the historical development of research on a particular topic or research question. It aims to provide a historical context for understanding the current state of knowledge on a particular topic.
• Problem-oriented literature review : This review focuses on a specific problem or issue and examines the literature to identify potential solutions or interventions. It aims to provide practical recommendations for addressing a particular problem or issue.
• Mixed-methods literature review : This type of review combines quantitative and qualitative methods to synthesize and analyze the available literature on a particular topic. It aims to provide a more comprehensive understanding of the research question by combining different types of evidence.

Parts of Literature Review

Parts of a literature review are as follows:

Introduction

The introduction of a literature review typically provides background information on the research topic and why it is important. It outlines the objectives of the review, the research question or hypothesis, and the scope of the review.

Literature Search

This section outlines the search strategy and databases used to identify relevant literature. The search terms used, inclusion and exclusion criteria, and any limitations of the search are described.

Literature Analysis

The literature analysis is the main body of the literature review. This section summarizes and synthesizes the literature that is relevant to the research question or hypothesis. The review should be organized thematically, chronologically, or by methodology, depending on the research objectives.

Critical Evaluation

Critical evaluation involves assessing the quality and validity of the literature. This includes evaluating the reliability and validity of the studies reviewed, the methodology used, and the strength of the evidence.

The conclusion of the literature review should summarize the main findings, identify any gaps in the literature, and suggest areas for future research. It should also reiterate the importance of the research question or hypothesis and the contribution of the literature review to the overall research project.

The references list includes all the sources cited in the literature review, and follows a specific referencing style (e.g., APA, MLA, Harvard).

How to write Literature Review

Here are some steps to follow when writing a literature review:

• Define your research question or topic : Before starting your literature review, it is essential to define your research question or topic. This will help you identify relevant literature and determine the scope of your review.
• Conduct a comprehensive search: Use databases and search engines to find relevant literature. Look for peer-reviewed articles, books, and other academic sources that are relevant to your research question or topic.
• Evaluate the sources: Once you have found potential sources, evaluate them critically to determine their relevance, credibility, and quality. Look for recent publications, reputable authors, and reliable sources of data and evidence.
• Organize your sources: Group the sources by theme, method, or research question. This will help you identify similarities and differences among the literature, and provide a structure for your literature review.
• Analyze and synthesize the literature : Analyze each source in depth, identifying the key findings, methodologies, and conclusions. Then, synthesize the information from the sources, identifying patterns and themes in the literature.
• Write the literature review : Start with an introduction that provides an overview of the topic and the purpose of the literature review. Then, organize the literature according to your chosen structure, and analyze and synthesize the sources. Finally, provide a conclusion that summarizes the key findings of the literature review, identifies gaps in knowledge, and suggests areas for future research.
• Edit and proofread: Once you have written your literature review, edit and proofread it carefully to ensure that it is well-organized, clear, and concise.

Examples of Literature Review

Here’s an example of how a literature review can be conducted for a thesis on the topic of “ The Impact of Social Media on Teenagers’ Mental Health”:

• Start by identifying the key terms related to your research topic. In this case, the key terms are “social media,” “teenagers,” and “mental health.”
• Use academic databases like Google Scholar, JSTOR, or PubMed to search for relevant articles, books, and other publications. Use these keywords in your search to narrow down your results.
• Evaluate the sources you find to determine if they are relevant to your research question. You may want to consider the publication date, author’s credentials, and the journal or book publisher.
• Begin reading and taking notes on each source, paying attention to key findings, methodologies used, and any gaps in the research.
• Organize your findings into themes or categories. For example, you might categorize your sources into those that examine the impact of social media on self-esteem, those that explore the effects of cyberbullying, and those that investigate the relationship between social media use and depression.
• Synthesize your findings by summarizing the key themes and highlighting any gaps or inconsistencies in the research. Identify areas where further research is needed.
• Use your literature review to inform your research questions and hypotheses for your thesis.

For example, after conducting a literature review on the impact of social media on teenagers’ mental health, a thesis might look like this:

“Using a mixed-methods approach, this study aims to investigate the relationship between social media use and mental health outcomes in teenagers. Specifically, the study will examine the effects of cyberbullying, social comparison, and excessive social media use on self-esteem, anxiety, and depression. Through an analysis of survey data and qualitative interviews with teenagers, the study will provide insight into the complex relationship between social media use and mental health outcomes, and identify strategies for promoting positive mental health outcomes in young people.”

Reference: Smith, J., Jones, M., & Lee, S. (2019). The effects of social media use on adolescent mental health: A systematic review. Journal of Adolescent Health, 65(2), 154-165. doi:10.1016/j.jadohealth.2019.03.024

Reference Example: Author, A. A., Author, B. B., & Author, C. C. (Year). Title of article. Title of Journal, volume number(issue number), page range. doi:0000000/000000000000 or URL

Applications of Literature Review

some applications of literature review in different fields:

• Social Sciences: In social sciences, literature reviews are used to identify gaps in existing research, to develop research questions, and to provide a theoretical framework for research. Literature reviews are commonly used in fields such as sociology, psychology, anthropology, and political science.
• Natural Sciences: In natural sciences, literature reviews are used to summarize and evaluate the current state of knowledge in a particular field or subfield. Literature reviews can help researchers identify areas where more research is needed and provide insights into the latest developments in a particular field. Fields such as biology, chemistry, and physics commonly use literature reviews.
• Health Sciences: In health sciences, literature reviews are used to evaluate the effectiveness of treatments, identify best practices, and determine areas where more research is needed. Literature reviews are commonly used in fields such as medicine, nursing, and public health.
• Humanities: In humanities, literature reviews are used to identify gaps in existing knowledge, develop new interpretations of texts or cultural artifacts, and provide a theoretical framework for research. Literature reviews are commonly used in fields such as history, literary studies, and philosophy.

Role of Literature Review in Research

Here are some applications of literature review in research:

• Identifying Research Gaps : Literature review helps researchers identify gaps in existing research and literature related to their research question. This allows them to develop new research questions and hypotheses to fill those gaps.
• Developing Theoretical Framework: Literature review helps researchers develop a theoretical framework for their research. By analyzing and synthesizing existing literature, researchers can identify the key concepts, theories, and models that are relevant to their research.
• Selecting Research Methods : Literature review helps researchers select appropriate research methods and techniques based on previous research. It also helps researchers to identify potential biases or limitations of certain methods and techniques.
• Data Collection and Analysis: Literature review helps researchers in data collection and analysis by providing a foundation for the development of data collection instruments and methods. It also helps researchers to identify relevant data sources and identify potential data analysis techniques.
• Communicating Results: Literature review helps researchers to communicate their results effectively by providing a context for their research. It also helps to justify the significance of their findings in relation to existing research and literature.

Purpose of Literature Review

Some of the specific purposes of a literature review are as follows:

• To provide context: A literature review helps to provide context for your research by situating it within the broader body of literature on the topic.
• To identify gaps and inconsistencies: A literature review helps to identify areas where further research is needed or where there are inconsistencies in the existing literature.
• To synthesize information: A literature review helps to synthesize the information from multiple sources and present a coherent and comprehensive picture of the current state of knowledge on the topic.
• To identify key concepts and theories : A literature review helps to identify key concepts and theories that are relevant to your research question and provide a theoretical framework for your study.
• To inform research design: A literature review can inform the design of your research study by identifying appropriate research methods, data sources, and research questions.

Characteristics of Literature Review

Some Characteristics of Literature Review are as follows:

• Identifying gaps in knowledge: A literature review helps to identify gaps in the existing knowledge and research on a specific topic or research question. By analyzing and synthesizing the literature, you can identify areas where further research is needed and where new insights can be gained.
• Establishing the significance of your research: A literature review helps to establish the significance of your own research by placing it in the context of existing research. By demonstrating the relevance of your research to the existing literature, you can establish its importance and value.
• Informing research design and methodology : A literature review helps to inform research design and methodology by identifying the most appropriate research methods, techniques, and instruments. By reviewing the literature, you can identify the strengths and limitations of different research methods and techniques, and select the most appropriate ones for your own research.
• Supporting arguments and claims: A literature review provides evidence to support arguments and claims made in academic writing. By citing and analyzing the literature, you can provide a solid foundation for your own arguments and claims.
• I dentifying potential collaborators and mentors: A literature review can help identify potential collaborators and mentors by identifying researchers and practitioners who are working on related topics or using similar methods. By building relationships with these individuals, you can gain valuable insights and support for your own research and practice.
• Keeping up-to-date with the latest research : A literature review helps to keep you up-to-date with the latest research on a specific topic or research question. By regularly reviewing the literature, you can stay informed about the latest findings and developments in your field.

Advantages of Literature Review

There are several advantages to conducting a literature review as part of a research project, including:

• Establishing the significance of the research : A literature review helps to establish the significance of the research by demonstrating the gap or problem in the existing literature that the study aims to address.
• Identifying key concepts and theories: A literature review can help to identify key concepts and theories that are relevant to the research question, and provide a theoretical framework for the study.
• Supporting the research methodology : A literature review can inform the research methodology by identifying appropriate research methods, data sources, and research questions.
• Providing a comprehensive overview of the literature : A literature review provides a comprehensive overview of the current state of knowledge on a topic, allowing the researcher to identify key themes, debates, and areas of agreement or disagreement.
• Identifying potential research questions: A literature review can help to identify potential research questions and areas for further investigation.
• Avoiding duplication of research: A literature review can help to avoid duplication of research by identifying what has already been done on a topic, and what remains to be done.
• Enhancing the credibility of the research : A literature review helps to enhance the credibility of the research by demonstrating the researcher’s knowledge of the existing literature and their ability to situate their research within a broader context.

Limitations of Literature Review

Limitations of Literature Review are as follows:

• Limited scope : Literature reviews can only cover the existing literature on a particular topic, which may be limited in scope or depth.
• Publication bias : Literature reviews may be influenced by publication bias, which occurs when researchers are more likely to publish positive results than negative ones. This can lead to an incomplete or biased picture of the literature.
• Quality of sources : The quality of the literature reviewed can vary widely, and not all sources may be reliable or valid.
• Time-limited: Literature reviews can become quickly outdated as new research is published, making it difficult to keep up with the latest developments in a field.
• Subjective interpretation : Literature reviews can be subjective, and the interpretation of the findings can vary depending on the researcher’s perspective or bias.
• Lack of original data : Literature reviews do not generate new data, but rather rely on the analysis of existing studies.
• Risk of plagiarism: It is important to ensure that literature reviews do not inadvertently contain plagiarism, which can occur when researchers use the work of others without proper attribution.

About the author

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The Literature Review: A Foundation for High-Quality Medical Education Research

a  These are subscription resources. Researchers should check with their librarian to determine their access rights.

Despite a surge in published scholarship in medical education 1 and rapid growth in journals that publish educational research, manuscript acceptance rates continue to fall. 2 Failure to conduct a thorough, accurate, and up-to-date literature review identifying an important problem and placing the study in context is consistently identified as one of the top reasons for rejection. 3 , 4 The purpose of this editorial is to provide a road map and practical recommendations for planning a literature review. By understanding the goals of a literature review and following a few basic processes, authors can enhance both the quality of their educational research and the likelihood of publication in the Journal of Graduate Medical Education ( JGME ) and in other journals.

The Literature Review Defined

In medical education, no organization has articulated a formal definition of a literature review for a research paper; thus, a literature review can take a number of forms. Depending on the type of article, target journal, and specific topic, these forms will vary in methodology, rigor, and depth. Several organizations have published guidelines for conducting an intensive literature search intended for formal systematic reviews, both broadly (eg, PRISMA) 5 and within medical education, 6 and there are excellent commentaries to guide authors of systematic reviews. 7 , 8

• A literature review forms the basis for high-quality medical education research and helps maximize relevance, originality, generalizability, and impact.
• A literature review provides context, informs methodology, maximizes innovation, avoids duplicative research, and ensures that professional standards are met.
• Literature reviews take time, are iterative, and should continue throughout the research process.
• Researchers should maximize the use of human resources (librarians, colleagues), search tools (databases/search engines), and existing literature (related articles).
• Keeping organized is critical.

Such work is outside the scope of this article, which focuses on literature reviews to inform reports of original medical education research. We define such a literature review as a synthetic review and summary of what is known and unknown regarding the topic of a scholarly body of work, including the current work's place within the existing knowledge . While this type of literature review may not require the intensive search processes mandated by systematic reviews, it merits a thoughtful and rigorous approach.

Purpose and Importance of the Literature Review

An understanding of the current literature is critical for all phases of a research study. Lingard 9 recently invoked the “journal-as-conversation” metaphor as a way of understanding how one's research fits into the larger medical education conversation. As she described it: “Imagine yourself joining a conversation at a social event. After you hang about eavesdropping to get the drift of what's being said (the conversational equivalent of the literature review), you join the conversation with a contribution that signals your shared interest in the topic, your knowledge of what's already been said, and your intention.” 9

The literature review helps any researcher “join the conversation” by providing context, informing methodology, identifying innovation, minimizing duplicative research, and ensuring that professional standards are met. Understanding the current literature also promotes scholarship, as proposed by Boyer, 10 by contributing to 5 of the 6 standards by which scholarly work should be evaluated. 11 Specifically, the review helps the researcher (1) articulate clear goals, (2) show evidence of adequate preparation, (3) select appropriate methods, (4) communicate relevant results, and (5) engage in reflective critique.

Failure to conduct a high-quality literature review is associated with several problems identified in the medical education literature, including studies that are repetitive, not grounded in theory, methodologically weak, and fail to expand knowledge beyond a single setting. 12 Indeed, medical education scholars complain that many studies repeat work already published and contribute little new knowledge—a likely cause of which is failure to conduct a proper literature review. 3 , 4

Likewise, studies that lack theoretical grounding or a conceptual framework make study design and interpretation difficult. 13 When theory is used in medical education studies, it is often invoked at a superficial level. As Norman 14 noted, when theory is used appropriately, it helps articulate variables that might be linked together and why, and it allows the researcher to make hypotheses and define a study's context and scope. Ultimately, a proper literature review is a first critical step toward identifying relevant conceptual frameworks.

Another problem is that many medical education studies are methodologically weak. 12 Good research requires trained investigators who can articulate relevant research questions, operationally define variables of interest, and choose the best method for specific research questions. Conducting a proper literature review helps both novice and experienced researchers select rigorous research methodologies.

Finally, many studies in medical education are “one-offs,” that is, single studies undertaken because the opportunity presented itself locally. Such studies frequently are not oriented toward progressive knowledge building and generalization to other settings. A firm grasp of the literature can encourage a programmatic approach to research.

Approaching the Literature Review

Considering these issues, journals have a responsibility to demand from authors a thoughtful synthesis of their study's position within the field, and it is the authors' responsibility to provide such a synthesis, based on a literature review. The aforementioned purposes of the literature review mandate that the review occurs throughout all phases of a study, from conception and design, to implementation and analysis, to manuscript preparation and submission.

Planning the literature review requires understanding of journal requirements, which vary greatly by journal ( table 1 ). Authors are advised to take note of common problems with reporting results of the literature review. Table 2 lists the most common problems that we have encountered as authors, reviewers, and editors.

Sample of Journals' Author Instructions for Literature Reviews Conducted as Part of Original Research Article a

Common Problem Areas for Reporting Literature Reviews in the Context of Scholarly Articles

Locating and Organizing the Literature

Three resources may facilitate identifying relevant literature: human resources, search tools, and related literature. As the process requires time, it is important to begin searching for literature early in the process (ie, the study design phase). Identifying and understanding relevant studies will increase the likelihood of designing a relevant, adaptable, generalizable, and novel study that is based on educational or learning theory and can maximize impact.

Human Resources

A medical librarian can help translate research interests into an effective search strategy, familiarize researchers with available information resources, provide information on organizing information, and introduce strategies for keeping current with emerging research. Often, librarians are also aware of research across their institutions and may be able to connect researchers with similar interests. Reaching out to colleagues for suggestions may help researchers quickly locate resources that would not otherwise be on their radar.

During this process, researchers will likely identify other researchers writing on aspects of their topic. Researchers should consider searching for the publications of these relevant researchers (see table 3 for search strategies). Additionally, institutional websites may include curriculum vitae of such relevant faculty with access to their entire publication record, including difficult to locate publications, such as book chapters, dissertations, and technical reports.

Strategies for Finding Related Researcher Publications in Databases and Search Engines

Search Tools and Related Literature

Researchers will locate the majority of needed information using databases and search engines. Excellent resources are available to guide researchers in the mechanics of literature searches. 15 , 16

Because medical education research draws on a variety of disciplines, researchers should include search tools with coverage beyond medicine (eg, psychology, nursing, education, and anthropology) and that cover several publication types, such as reports, standards, conference abstracts, and book chapters (see the box for several information resources). Many search tools include options for viewing citations of selected articles. Examining cited references provides additional articles for review and a sense of the influence of the selected article on its field.

Box Information Resources

• Web of Science a
• Education Resource Information Center (ERIC)
• Cumulative Index of Nursing & Allied Health (CINAHL) a
• Google Scholar

Once relevant articles are located, it is useful to mine those articles for additional citations. One strategy is to examine references of key articles, especially review articles, for relevant citations.

Getting Organized

As the aforementioned resources will likely provide a tremendous amount of information, organization is crucial. Researchers should determine which details are most important to their study (eg, participants, setting, methods, and outcomes) and generate a strategy for keeping those details organized and accessible. Increasingly, researchers utilize digital tools, such as Evernote, to capture such information, which enables accessibility across digital workspaces and search capabilities. Use of citation managers can also be helpful as they store citations and, in some cases, can generate bibliographies ( table 4 ).

Citation Managers

Knowing When to Say When

Researchers often ask how to know when they have located enough citations. Unfortunately, there is no magic or ideal number of citations to collect. One strategy for checking coverage of the literature is to inspect references of relevant articles. As researchers review references they will start noticing a repetition of the same articles with few new articles appearing. This can indicate that the researcher has covered the literature base on a particular topic.

Putting It All Together

In preparing to write a research paper, it is important to consider which citations to include and how they will inform the introduction and discussion sections. The “Instructions to Authors” for the targeted journal will often provide guidance on structuring the literature review (or introduction) and the number of total citations permitted for each article category. Reviewing articles of similar type published in the targeted journal can also provide guidance regarding structure and average lengths of the introduction and discussion sections.

When selecting references for the introduction consider those that illustrate core background theoretical and methodological concepts, as well as recent relevant studies. The introduction should be brief and present references not as a laundry list or narrative of available literature, but rather as a synthesized summary to provide context for the current study and to identify the gap in the literature that the study intends to fill. For the discussion, citations should be thoughtfully selected to compare and contrast the present study's findings with the current literature and to indicate how the present study moves the field forward.

To facilitate writing a literature review, journals are increasingly providing helpful features to guide authors. For example, the resources available through JGME include several articles on writing. 17 The journal Perspectives on Medical Education recently launched “The Writer's Craft,” which is intended to help medical educators improve their writing. Additionally, many institutions have writing centers that provide web-based materials on writing a literature review, and some even have writing coaches.

The literature review is a vital part of medical education research and should occur throughout the research process to help researchers design a strong study and effectively communicate study results and importance. To achieve these goals, researchers are advised to plan and execute the literature review carefully. The guidance in this editorial provides considerations and recommendations that may improve the quality of literature reviews.

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• Systematic Review
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• Published: 24 June 2024

Placebo effects in randomized trials of pharmacological and neurostimulation interventions for mental disorders: An umbrella review

• Nathan T. M. Huneke   ORCID: orcid.org/0000-0001-5981-6707 1 , 2 ,
• Jay Amin   ORCID: orcid.org/0000-0003-3792-0428 1 , 2 ,
• David S. Baldwin 1 , 2 , 3 ,
• Alessio Bellato 4 , 5 ,
• Valerie Brandt   ORCID: orcid.org/0000-0002-3208-2659 5 , 6 ,
• Samuel R. Chamberlain 1 , 2 ,
• Christoph U. Correll   ORCID: orcid.org/0000-0002-7254-5646 7 , 8 , 9 , 10 ,
• Luis Eudave 11 ,
• Matthew Garner 1 , 5 , 12 ,
• Corentin J. Gosling 5 , 13 , 14 ,
• Catherine M. Hill 1 , 15 ,
• Ruihua Hou 1 ,
• Oliver D. Howes   ORCID: orcid.org/0000-0002-2928-1972 16 , 17 , 18 ,
• Konstantinos Ioannidis 1 , 2 ,
• Ole Köhler-Forsberg 19 , 20 ,
• Lucia Marzulli 21 ,
• Claire Reed   ORCID: orcid.org/0000-0003-1385-4729 5 ,
• Julia M. A. Sinclair 1 ,
• Satneet Singh 2 ,
• Marco Solmi   ORCID: orcid.org/0000-0003-4877-7233 5 , 22 , 23 , 24 , 25   na1 &
• Samuele Cortese   ORCID: orcid.org/0000-0001-5877-8075 1 , 5 , 26 , 27 , 28   na1  

Molecular Psychiatry ( 2024 ) Cite this article

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• Psychiatric disorders

There is a growing literature exploring the placebo response within specific mental disorders, but no overarching quantitative synthesis of this research has analyzed evidence across mental disorders. We carried out an umbrella review of meta-analyses of randomized controlled trials (RCTs) of biological treatments (pharmacotherapy or neurostimulation) for mental disorders. We explored whether placebo effect size differs across distinct disorders, and the correlates of increased placebo effects. Based on a pre-registered protocol, we searched Medline, PsycInfo, EMBASE, and Web of Knowledge up to 23.10.2022 for systematic reviews and/or meta-analyses reporting placebo effect sizes in psychopharmacological or neurostimulation RCTs. Twenty meta-analyses, summarising 1,691 RCTs involving 261,730 patients, were included. Placebo effect size varied, and was large in alcohol use disorder ( g  = 0.90, 95% CI [0.70, 1.09]), depression ( g  = 1.10, 95% CI [1.06, 1.15]), restless legs syndrome ( g  = 1.41, 95% CI [1.25, 1.56]), and generalized anxiety disorder ( d  = 1.85, 95% CI [1.61, 2.09]). Placebo effect size was small-to-medium in obsessive-compulsive disorder ( d  = 0.32, 95% CI [0.22, 0.41]), primary insomnia ( g  = 0.35, 95% CI [0.28, 0.42]), and schizophrenia spectrum disorders (standardized mean change = 0.33, 95% CI [0.22, 0.44]). Correlates of larger placebo response in multiple mental disorders included later publication year (opposite finding for ADHD), younger age, more trial sites, larger sample size, increased baseline severity, and larger active treatment effect size. Most (18 of 20) meta-analyses were judged ‘low’ quality as per AMSTAR-2. Placebo effect sizes varied substantially across mental disorders. Future research should explore the sources of this variation. We identified important gaps in the literature, with no eligible systematic reviews/meta-analyses of placebo response in stress-related disorders, eating disorders, behavioural addictions, or bipolar mania.

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Introduction.

A placebo is an ‘inactive’ substance or ‘sham’ technique that is used as a control for assessing the efficacy of an active treatment [ 1 ]. However, study participants in a placebo control group may experience considerable symptom improvements - a ‘placebo response’ [ 1 , 2 , 3 ]. Statistical artifacts or non-specific effects account for some of the placebo response. For example, many individuals seek treatment and are enrolled in clinical trials while their symptoms are at their worst. Their symptoms will gradually return to their usual severity (‘regression to the mean’), giving the appearance of a placebo response [ 4 ]. Further, it has been suggested that the placebo response is exacerbated due to unreliable ratings as well as baseline symptom severity inflation if raters are aware of severity criteria for entry to a trial [ 5 , 6 ]. Other potential sources of apparent placebo responses include sampling biases caused by the withdrawal of the least improved patients in the placebo arm, non-specific beneficial effects resulting from interactions with staff delivering the trial, environmental effects due to inpatient care during placebo-controlled trials, or other unaccounted for factors, such as dietary or exercise changes during the trial [ 7 , 8 , 9 ]. Nonetheless, there is evidence that placebo administration results in ‘true’ - or non-artefactual - placebo effects, that is, identifiable changes in biological systems [ 1 , 10 , 11 ]. For example, placebo administration is capable of causing immunosuppression [ 12 , 13 ], placebo effects in Parkinson’s disease are driven by striatal dopamine release [ 10 , 14 ], and placebo analgesia is mediated by endogenous opioid release [ 15 , 16 ]. Furthermore, there is evidence that placebo effects in depressive and anxiety disorders are correlated with altered activity in the ventral striatum, orbitofrontal cortex, rostral anterior cingulate cortex, and the default mode network [ 17 ]. The placebo effect size can be increased through the use of verbal suggestions and conditioning procedures, thus suggesting the underlying role of psychological mechanisms including learning and expectations [ 11 , 18 ].

Across age groups, treatment modalities, and diverse mental disorders, biological treatments (pharmacotherapy or neurostimulation) do reduce symptoms [ 19 , 20 , 21 , 22 ], but only a subgroup of patients experience a clinically significant symptom response or enter remission [ 23 , 24 , 25 ]. Furthermore, current medications may also have unfavourable side effects [ 23 , 26 , 27 , 28 , 29 , 30 , 31 ]. Given the high prevalence of mental disorders and their significant socioeconomic burden [ 32 , 33 , 34 ], there is a need to develop more effective and safer psychopharmacologic and neurostimulation treatments. However, in randomized-controlled trials (RCTs), the magnitude of the placebo response may be considerable, which can affect the interpretation of their results [ 35 , 36 , 37 ]. For example, in antipsychotic trials over the past 40 years, placebo response has increased while medication response has remained consistent [ 38 , 39 ]. Consequently, the trial’s ability to statistically differentiate between an active medication and a placebo is diminished [ 40 ]. Indeed, large placebo response rates have been implicated in hindering psychotropic drug development [ 41 , 42 ]. The increased placebo response can also affect larger data synthesis approaches, such as network meta-analysis, in which assumptions about placebo responses (e.g. stability over time) might affect the validity of results [ 43 ].

Improved understanding of participant, trial, and mental disorder-related factors that contribute to placebo response might allow better clinical trial design to separate active treatment from placebo effects. There is a growing body of research, including individual studies and systematic reviews/meta-analyses, examining the placebo response within specific mental disorders [ 35 ]. However, to date, no overarching synthesis of this literature, to detect any similarities or differences across mental disorders, has been published. We therefore carried out an umbrella review of meta-analyses to address this need. We aimed to assess the placebo effect size in RCTs for a range of mental disorders, whether the effect size differs across distinct mental disorders, and identify any correlates of increased placebo effect size or response rate.

The protocol for this systematic umbrella review was pre-registered on the open science framework ( https://osf.io/fxvn4/ ) and published [ 44 ]. Deviations from this protocol, and additions to it, were: eight authors were involved in record screening rather than two; we reported effect sizes pooled across age groups and analyses comparing placebo effect sizes between age groups; and we included a meta-analysis that incorporated trials of dietary supplements as well as medications in autism. For the rationale behind these decisions, see eMethods.

Eight authors (NH, AB, VB, LE, OKF, LM, CR, SS) carried out the systematic review and data extraction independently in pairs. Discrepancies were resolved through consensus or through arbitration by a third reviewer (NH or SCo). We searched, without date or language restrictions, up to 23.10.2022, Medline, PsycInfo, EMBASE + EMBASE Classic, and Web of Knowledge for systematic reviews with or without meta-analyses of RCTs of biological treatments (psychopharmacotherapy or neurostimulation) compared with a placebo or sham treatment in individuals with mental disorders diagnosed according to standardized criteria. The full search strategy is included in eMethods. We also sought systematic reviews of RCTs conducted in patients with sleep-wake disorders, since these disorders are included in the DSM-5 and their core symptoms overlap with those of mental disorders [ 45 ]. We retained systematic reviews with or without meta-analyses that reported within-group changes in symptoms in the placebo arm.

Next, to prevent duplication of data, a matrix containing all eligible systematic reviews/meta-analyses for each category of mental disorder was created. Where there were multiple eligible systematic reviews/meta-analyses for the same disorder and treatment, we preferentially included meta-analyses, and if multiple eligible meta-analyses remained, then we included the one containing the largest number of studies for the same disorder and treatment, in line with recent umbrella reviews [ 46 , 47 ].

Data were extracted by at least two among six reviewers (AB, VB, LE, OKF, CR, SS) independently in pairs via a piloted form. All extracted data were further checked by a third reviewer (NH). See eMethods for a list of extracted data.

Our primary outcome was the pre-post effect size of the placebo/sham related to the condition-specific primary symptom change for each mental disorder. Secondary outcomes included any other reported clinical outcomes in eligible reviews. We report effect sizes calculated within-group from baseline and post-treatment means by meta-analysis authors, including Cohen’s d and Hedges’ g for repeated measures, which account for both mean difference and correlation between paired observations; and standardized mean change, where the average change score is divided by standard deviation of the change scores. We interpreted the effect size in line with the suggestion by Cohen [ 48 ], i.e. small (~0.2), medium (~0.5), or large (~0.8).

In addition, we extracted data regarding potential correlates of increased placebo effect size or response rate (as defined and assessed by the authors of each meta-analysis) in each mental disorder identified through correlation analyses or meta-regression. Where available, results from multivariate analyses were preferred.

The methodological quality of included reviews was assessed by at least two among six reviewers (AB, VB, LE, OKF, NH, CR) independently and in pairs using the AMSTAR-2 tool, a critical appraisal tool that enables reproducible assessments of the conduct of systematic reviews [ 49 ]. The methodological quality of each included review was rated as high, moderate, low, or critically low.

Our initial search identified 6,108 records. After screening titles and abstracts, we obtained and assessed 115 full-text reports (see eResults for a list of articles excluded following full-text assessment, with reasons). Of these, 20 were deemed eligible, and all were systematic reviews with meta-analysis (Fig.  1 ). In total, the 20 included meta-analyses synthesized data from 1,691 RCTs (median 55) involving 261,730 patients (median 5,365). These meta-analyses were published between 2007 and 2022 and involved individuals with the following mental disorders: major depressive disorder (MDD; n  = 6) [ 50 , 51 , 52 , 53 , 54 , 55 ], anxiety disorders ( n  = 4) [ 55 , 56 , 57 , 58 ], schizophrenia spectrum disorders ( n  = 3) [ 38 , 59 , 60 ], alcohol use disorder (AUD; n  = 1) [ 61 ], attention-deficit/hyperactivity disorder (ADHD; n  = 1) [ 62 ], autism spectrum disorders ( n  = 1) [ 63 ], bipolar depression ( n  = 1) [ 64 ], intellectual disability ( n  = 1) [ 65 ], obsessive-compulsive disorder (OCD; n  = 1) [ 66 ], primary insomnia ( n  = 1) [ 67 ], and restless legs syndrome (RLS; n  = 1) [ 68 ].

Twenty meta-analyses were included.

The methodological quality of the included meta-analyses according to AMSTAR-2 ratings was high in two meta-analyses (ADHD and autism), low in four meta-analyses, and critically low in the remaining 14 meta-analyses (Table  1 ). The most common sources of bias that led to downgrading on the AMSTAR-2 were: no list of excluded full-text articles with reasons ( k  = 14), no explicit statement that the protocol was pre-registered ( k  = 14), and no assessment of the potential impact of risk of bias in individual studies on the results ( k  = 13). The full reasoning behind our AMSTAR-2 ratings is included in eResults.

Our first objective was to determine placebo effect sizes across mental conditions. Data regarding within-group placebo efficacy were reported in sixteen of the included meta-analyses [ 38 , 50 , 52 , 53 , 55 , 56 , 57 , 58 , 60 , 61 , 62 , 63 , 65 , 66 , 67 , 68 ]. Placebo effect sizes for the primary outcomes ranged from 0.23 to 1.85, with a median of 0.64 (Fig.  2 ). Median heterogeneity across meta-analyses was I 2  = 72%, suggesting a generally high percentage of heterogeneity due to true variation across studies.

Dots represent placebo group effect size while triangles represent active effect size. CI confidence interval, MDD major depressive disorder, GAD generalized anxiety disorder, SAD social anxiety disorder, OCD obsessive-compulsive disorder, g Hedges’ g, d Cohen’s d, SMC standardized mean change, NR not reported.

A detailed description of each meta-analysis included for this objective is included in eResults. Here, we report a summary of these results in order of the greatest number of RCT’s and meta-analyses included per disorder. In MDD, a large within-group placebo effect was observed ( g  = 1.10, 95% CI [1.06, 1.15]), although active medication had an even larger effect size ( g  = 1.49, 95% CI [1.44, 1.53]) [ 50 ]. Similarly, in children and adolescents with MDD, placebo effect size was large ( g  = 1.57, 95% CI [1.36, 1.78]), as was serotonergic medication effect size ( g  = 1.85, 95% CI [1.70, 2.00]) [ 55 ]. In treatment-resistant MDD, the within-group placebo effect size was smaller than in non-treatment-resistant MDD ( g  = 0.89, 95% CI [0.81, 0.98]) [ 52 ]. In neuromodulation trials for MDD, the effect size of sham was g  = 0.80 (95% CI [0.65, 0.95]) [ 53 ]. In this meta-analysis, the effect size was larger for non-treatment-resistant ( g  = 1.28, 95% CI [0.47, 2.97]) compared to treatment-resistant participants (g = 0.50 95% CI [0.03, 0.99]) [ 53 ]. In adults with anxiety disorders, placebo effect sizes varied across disorders, with a medium effect size in panic disorder ( d  = 0.57, 95% CI [0.50, 0.64]) [ 56 ] and large effect sizes in generalized anxiety disorder (GAD) ( d  = 1.85, 95% CI [1.61, 2.09]) and social anxiety disorder (SAD) ( d  = 0.94, 95% CI [0.77, 1.12]) [ 57 ]. Other meta-analyses in children and adolescents and older adults pooled RCTs across anxiety disorders, and found large placebo effect sizes ( g  = 1.03, 95% CI [0.84, 1.21] and d  = 1.06, 95% CI [0.71, 1.42], respectively) [ 55 , 58 ]. In ADHD, placebo effect size was medium-to-large for clinician-rated outcomes (SMC = 0.75, 95% CI [0.67, 0.83]) [ 62 ]. There was additionally a significant negative relationship between placebo effect size and drug-placebo difference (−0.56, p  < 0.01) for self-rated outcomes [ 62 ]. In schizophrenia spectrum disorders, placebo effect size was small-to-medium in antipsychotic RCTs (SMC = 0.33, 95% CI [0.22, 0.44]) [ 38 ] and medium in RCTs focusing specifically on negative symptoms ( d  = 0.64, 95% CI [0.46, 0.83]) [ 60 ]. Placebo effect size in RLS was large when measured via rating scales ( g  = 1.41, 95% CI [1.25, 1.56]), but small ( g  = 0.02 to 0.24) in RCTs using objective outcomes [ 68 ]. In autism, placebo effect sizes were small (SMC ranged 0.23 to 0.36) [ 63 ]. Similarly, placebo effect size was small in OCD ( d  = 0.32, 95% CI [0.22, 0.41]), although larger in children and adolescents ( d  = 0.45, 95% CI [0.35, 0.56]) compared with adults ( d  = 0.27, 95% CI [0.15, 0.38]) [ 66 ]. Placebo effect size was large in AUD ( g  = 0.90, 95% CI [0.70, 1.09]) [ 61 ], small in primary insomnia ( g ranged 0.25 to 0.43) [ 67 ], and medium in intellectual disability related to genetic causes ( g  = 0.47, 95% CI [0.18, 0.76]) [ 65 ].

Our second objective was to examine the correlates of increased placebo response. We included 14 meta-analyses that reported correlates of placebo effect size or response rate through correlation analysis or meta-regression [ 38 , 51 , 53 , 54 , 56 , 57 , 59 , 60 , 61 , 62 , 63 , 64 , 66 , 68 ]. The key correlates extracted from these studies are summarized in Table  2 .

Several variables were consistently identified across meta-analyses. Increased number of trial sites was a positive correlate of increased placebo response in MDD [ 51 , 54 ], schizophrenia spectrum disorders [ 59 ], and autism spectrum disorders [ 63 ]. Similarly, increased sample size was positively associated with placebo effect size in schizophrenia spectrum disorders [ 59 ], OCD [ 66 ], and panic disorder [ 56 ]. Later publication or study year was associated with greater placebo response in anxiety disorders [ 56 , 57 ], schizophrenia spectrum disorders [ 38 ], AUD [ 61 ], and OCD [ 66 ] but not in MDD [ 51 ], and with reduced placebo response in ADHD [ 62 ]. Younger age was associated with increased placebo responses in schizophrenia spectrum disorders [ 38 , 59 ] and OCD [ 66 ]. Increased baseline illness severity was associated with increased placebo response in schizophrenia spectrum disorders [ 38 ], ADHD [ 62 ], and AUD [ 61 ]. Increased trial or follow-up duration was positively associated with increased placebo response in MDD [ 51 ], but negatively associated with placebo response in schizophrenia spectrum disorders [ 38 , 60 ] and OCD [ 66 ]. Finally, the effect size of active treatment was positively associated with increased placebo response in neurostimulation trials for MDD [ 53 ], bipolar depression [ 64 ], autistic spectrum disorders [ 63 ], and ADHD [ 62 ].

There were also some variables associated with increased placebo response in single disorders only. Flexible dosing, rather than fixed dosing, was associated with increased placebo response in MDD [ 51 ]. Increased illness duration was associated with reduced placebo response in schizophrenia spectrum disorders [ 38 ]. In RCTs for negative symptoms of schizophrenia, a higher number of active treatment arms was associated with increased placebo response [ 60 ]. A number of treatment administrations was a positive correlate of increased placebo response in patients with AUD [ 61 ]. A low risk of bias in selective reporting was associated with increased placebo response in ADHD [ 62 ]. Finally, a low risk of bias in allocation concealment was associated with increased placebo response in autism [ 63 ].

To our knowledge, this is the first overarching synthesis of the literature exploring the placebo response in RCTs of biological treatments across a broad range of mental disorders. We found that placebo responses were present and detectable across mental disorders. Further, the placebo effect size across these disorders varied between small and large (see Fig.  3 ). Additionally, several variables appeared to be associated with increased placebo effect size or response rate across a number of disorders, while others were reported for individual disorders only.

CI confidence interval, MDD major depressive disorder, GAD generalized anxiety disorder, SAD social anxiety disorder, OCD obsessive-compulsive disorder, g Hedges’ g, d Cohen’s d, SMC standardized mean change.

Our umbrella review distinguishes itself from a recent publication on placebo mechanisms across medical conditions [ 69 ]. Only four systematic reviews of research in mental disorders were included in that recent review [ 69 ], none of which were eligible for inclusion in our umbrella review, as we focus specifically on RCTs in mental disorders. Thus, our current umbrella review synthesizes different literature and is complementary [ 69 ].

We found substantial variation in placebo effect sizes across mental disorders. In GAD, SAD, MDD, AUD, and RLS (for subjective outcomes), placebo effects were large (>0.9), while they were small (approximately 0.3) in OCD, primary insomnia, autism, RLS (for objective outcomes), and schizophrenia spectrum disorders. It is noteworthy that placebo effect size/response rate correlated with active treatment effect size/response rate in many disorders (MDD, bipolar depression, ADHD, and autism). Nonetheless, where reported, active treatment was always superior. This possibly suggests an underlying ‘treatment responsiveness’ of these disorders that can vary in size. Perhaps, the natural history of a disorder is an important factor in ‘responsiveness’, i.e., disorders in which there is greater natural fluctuation in severity will show larger placebo (and active treatment) effect sizes. Supporting this hypothesis, increased trial duration predicted a larger placebo effect size in MDD, a disorder in which the natural course includes improvement [ 31 , 51 , 70 ]. Conversely, in schizophrenia spectrum disorders where improvement (particularly of negative symptoms) is less likely [ 71 ], increased trial and illness duration predicted a smaller placebo effect size [ 38 , 60 ]. However, previous meta-analyses suggest that natural improvement, for example, measured via waiting list control, does not fully account for the placebo effect in depression and anxiety disorders [ 72 , 73 ]. Statistical artifact, therefore, does not seem to fully explain the variation in effect size.

Non-specific treatment mechanisms are likely an additional source of the observed placebo effect. For example, those with treatment-resistant illness might have reduced expectations regarding treatment. This assumption is supported by the subgroup analysis reported by Razza and colleagues showing sham neuromodulation efficacy reduced as the number of previous failed antidepressant trials increased [ 53 ]. Another factor to consider is the outcome measure chosen. For example, the placebo effect size in panic disorder was smaller when calculated with objective or self-report measures compared with clinician-rated measures [ 56 ]. A similar finding was reported in ADHD trials [ 62 ]. Why placebo effect sizes would differ with clinician-rated versus self-rated scales is unclear. This might result from ‘demand characteristics’ (i.e., cues that suggest to a patient how they ‘should’ respond), or unblinding of the rater, or a combination of the two [ 74 , 75 ].

Several correlates of increased placebo response were reported in included meta-analyses. These included a larger sample size, more study sites, a later publication year (but with an opposite finding for ADHD), younger age, and increased baseline illness severity. This might reflect changes in clinical trial methods over time, the potential for increased ‘noise’ in the data with larger samples or more study sites, and, more speculatively, variables associated with increased volatility in symptoms [ 39 , 51 , 76 ]. A more extensive discussion regarding the potential reasons these variables might correlate with, or predict, placebo response is included in the eDiscussion. Although some correlates of increased placebo response were identified, perhaps more pertinently, it is unknown whether these also predict the separation between active treatment and placebo in most mental disorders. Three included meta-analyses did show that as placebo response increases, the likelihood of drug-placebo separation decreases [ 38 , 62 , 64 ]. This suggests correlates of placebo effect size are also correlates of trial success or failure, but this hypothesis needs explicit testing. In addition, few of the meta-analyses we included explored whether correlates of placebo response differed from correlates of active treatment response. For example, in clinical trials for gambling disorder, response to active treatment was predicted by weeks spent in the trial and by baseline severity, while response to placebo was predicted by baseline depressive and anxiety symptoms [ 77 ]. Furthermore, there is evidence that industry sponsorship is a specific correlate of reduced drug-placebo separation in schizophrenia spectrum disorders [ 78 ]. The largest meta-analysis that we included (conducted by Scott et al. [ 50 ]) did not explore correlates of increased placebo response through meta-regression analysis; rather, it was designed specifically to assess the impact of the use of placebo run-in periods in antidepressant trials. The authors found that use of a placebo run-in was associated with reduced placebo response. However, this effect did not enhance sensitivity to detect medication efficacy versus control groups, as trials with placebo run-in periods were also associated with a reduced medication response. Similar effects of placebo run-in were seen in univariate (but not multivariable) models in ADHD, where placebo run-in reduced placebo effect size in youth, but did not affect drug vs placebo difference [ 62 ]. Further work should be undertaken to ascertain whether trial-level correlates (including the use of placebo run-in) differentially explain active treatment or placebo response and whether controlling for these can improve drug-placebo separation.

Our results should be considered in the light of several possible limitations. First, as in any umbrella review, we were limited by the quality of the meta-analyses we included. Our AMSTAR-2 ratings suggest that confidence in the conclusions of most included meta-analyses should be critically low or low. Indeed, several meta-analyses did not assess for publication bias or for bias in included RCTs. This is relevant, as the risk of bias in selective reporting was highlighted as potentially being associated with placebo effect size in ADHD [ 62 ], and might therefore be relevant in other mental disorders. Second, our results are potentially vulnerable to biases or unmeasured confounders present in the included meta-analyses. Third, we attempted to prevent overlap and duplication of information by including only the meta-analyses with the most information. This might, however, have resulted in some data not being included in our synthesis. Fourth, an exploration of the potential clinical relevance of the placebo effect sizes reported here was outside the scope of the current review but should be considered an important question for future research. Finally, the meta-analyses we included encompassed RCTs with different levels of blinding (double-blind, single-blind). Although the majority of trials were likely double-blind, it is possible that different levels of blinding could have influenced placebo effect sizes through effects on expectations. Future analyses of placebo effects and their correlates should either focus on double-blind trials or compare results across levels of blinding. Related to this, the included meta-analyses pooled phase 2 and phase 3 trials (the latter of which will usually follow positive phase 2 trials), which might result in different expectation biases. Therefore, placebo effects should be compared between phase 2 and phase 3 trials in the future.

In this umbrella review, we found placebo effect sizes varied substantially across mental disorders. The sources of this variation remain unknown and require further study. Some variables were correlates of increased placebo response across mental disorders, including larger sample size, higher number of study sites, later publication year (opposite for ADHD), younger age, and increased baseline illness severity. There was also evidence that clinician-rated outcomes were associated with larger placebo effect sizes than self-rated or objective outcomes. We additionally identified important gaps in the literature, with no eligible systematic reviews identified in stress-related disorders, eating disorders, behavioural addictions, or bipolar mania. In relation to these disorders, some analyses have been published but they have not been included in systematic reviews/meta-analyses (e.g. analyses of individual patient data pooled across RCTs in acute mania [ 79 ] or gambling disorder [ 77 , 80 ]) and therefore were not eligible for inclusion here. We also focused on placebo response in RCTs of pharmacotherapies and neurostimulation interventions for mental disorders. We did not include placebo effects in psychosocial interventions, but such an analysis would also be valuable. Future studies should address these gaps in the literature and furthermore should compare findings in placebo arms with active treatment arms, both regarding treatment effect size and its correlates. Gaining additional insights into the placebo response may improve our ability to separate active treatment effects from placebo effects, thus paving the way for potentially effective new treatments for mental disorders.

Data availability

The datasets generated during and/or analysed during the current study are available in the Open Science Framework repository, https://osf.io/fxvn4/ .

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Acknowledgements

Dr Nathan TM Huneke is an NIHR Academic Clinical Lecturer. The views expressed in this publication are those of the author(s) and not necessarily those of the NIHR, NHS, or the UK Department of Health and Social Care. For the purpose of open access, the author has applied a Creative Commons Attribution License (CC BY) to any Author Accepted Manuscript version arising from this submission.

Author contributors

NTMH, JA, DSB, SRC, CUC, MG, CMH, RH, ODH, JMAS, MS, and SCo conceptualized the study. NTMH, AB, VB, LE, CJG, OKF, LM, CR, SS, and SCo contributed to data collection, data curation, or data analysis. NTMH, MS, and SCo wrote the first draft of the manuscript. All authors had access to the raw data. All authors reviewed and edited the manuscript and had final responsibility for the decision to submit it for publication.

Author information

These authors contributed equally: Marco Solmi, Samuele Cortese.

Authors and Affiliations

Clinical and Experimental Sciences, Faculty of Medicine, University of Southampton, Southampton, UK

Nathan T. M. Huneke, Jay Amin, David S. Baldwin, Samuel R. Chamberlain, Matthew Garner, Catherine M. Hill, Ruihua Hou, Konstantinos Ioannidis, Julia M. A. Sinclair & Samuele Cortese

Southern Health NHS Foundation Trust, Southampton, UK

Nathan T. M. Huneke, Jay Amin, David S. Baldwin, Samuel R. Chamberlain, Konstantinos Ioannidis & Satneet Singh

University Department of Psychiatry and Mental Health, University of Cape Town, Cape Town, South Africa

David S. Baldwin

School of Psychology, University of Nottingham Malaysia, Semenyih, Malaysia

Alessio Bellato

Centre for Innovation in Mental Health, School of Psychology, Faculty of Environmental and Life Sciences, University of Southampton, Southampton, UK

Alessio Bellato, Valerie Brandt, Matthew Garner, Corentin J. Gosling, Claire Reed, Marco Solmi & Samuele Cortese

Clinic of Psychiatry, Social Psychiatry and Psychotherapy, Hannover Medical School, Hanover, Germany

Valerie Brandt

Department of Child and Adolescent Psychiatry, Charité Universitätsmedizin Berlin, Berlin, Germany

Christoph U. Correll

Department of Psychiatry, Zucker Hillside Hospital, Northwell Health, Glen Oaks, NY, USA

Department of Psychiatry and Molecular Medicine, Zucker School of Medicine at Hofstra/Northwell, Hempstead, NY, USA

Center for Psychiatric Neuroscience, Feinstein Institute for Medical Research, Manhasset, NY, USA

Faculty of Education and Psychology, University of Navarra, Pamplona, Spain

Luis Eudave

School of Psychology, Faculty of Environmental and Life Sciences, University of Southampton, Southampton, UK

Matthew Garner

Université Paris Nanterre, DysCo Lab, F-92000, Nanterre, France

Corentin J. Gosling

Université de Paris, Laboratoire de Psychopathologie et Processus de Santé, F-92100, Boulogne-Billancourt, France

Department of Sleep Medicine, Southampton Children’s Hospital, Southampton, UK

Catherine M. Hill

Department of Psychosis Studies, Institute of Psychiatry, Psychology & Neuroscience, King’s College London, London, UK

Oliver D. Howes

H Lundbeck A/s, Iveco House, Watford, UK

Institute of Clinical Sciences (ICS), Faculty of Medicine, Imperial College London, London, UK

Department of Clinical Medicine, Aarhus University, Aarhus, Denmark

Ole Köhler-Forsberg

Psychosis Research Unit, Aarhus University Hospital–Psychiatry, Aarhus, Denmark

Department of Translational Biomedicine and Neuroscience (DIBRAIN), University of Studies of Bari “Aldo Moro”, Bari, Italy

Lucia Marzulli

Department of Psychiatry, University of Ottawa, Ottawa, ON, Canada

Marco Solmi

Department of Mental Health, Ottawa Hospital, Ottawa, ON, Canada

Ottawa Hospital Research Institute (OHRI) Clinical Epidemiology Program, University of Ottawa, Ottawa, ON, Canada

School of Epidemiology and Public Health, Faculty of Medicine, University of Ottawa, Ottawa, ON, Canada

Solent NHS Trust, Southampton, UK

Samuele Cortese

DiMePRe-J-Department of Precision and Regenerative Medicine-Jonic Area, University “Aldo Moro”, Bari, Italy

Hassenfeld Children’s Hospital at NYU Langone, New York University Child Study Center, New York, NY, USA

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DSB is President of the British Association for Psychopharmacology, Editor of the Human Psychopharmacology journal (for which he receives an editor’s honorarium), and has received royalties from UpToDate. CMH has acted on an expert advisory board for Neurim Pharmaceuticals. ODH is a part-time employee and stockholder of Lundbeck A/s. He has received investigator-initiated research funding from and/or participated in advisory/speaker meetings organized by Angellini, Autifony, Biogen, Boehringer-Ingelheim, Eli Lilly, Heptares, Global Medical Education, Invicro, Jansenn, Lundbeck, Neurocrine, Otsuka, Sunovion, Recordati, Roche and Viatris/Mylan. ODH has a patent for the use of dopaminergic imaging. All other authors declare no competing interests. MS has received honoraria/has been a consultant for Angelini, Lundbeck, and Otsuka. SCo has received honoraria from non-profit associations (BAP, ACAMH, CADDRA) for educational activities and an honorarium from Medice. KI has received honoraria from Elsevier for editorial work. SRC receives honoraria from Elsevier for associate editor roles at comprehensive psychiatry and NBR journals. CUC has been a consultant and/or advisor to or has received honoraria from: AbbVie, Acadia, Adock Ingram, Alkermes, Allergan, Angelini, Aristo, Biogen, Boehringer-Ingelheim, Bristol-Meyers Squibb, Cardio Diagnostics, Cerevel, CNX Therapeutics, Compass Pathways, Darnitsa, Denovo, Gedeon Richter, Hikma, Holmusk, IntraCellular Therapies, Jamjoom Pharma, Janssen/J&J, Karuna, LB Pharma, Lundbeck, MedAvante-ProPhase, MedInCell, Merck, Mindpax, Mitsubishi Tanabe Pharma, Mylan, Neurocrine, Neurelis, Newron, Noven, Novo Nordisk, Otsuka, Pharmabrain, PPD Biotech, Recordati, Relmada, Reviva, Rovi, Sage, Seqirus, SK Life Science, Sumitomo Pharma America, Sunovion, Sun Pharma, Supernus, Takeda, Teva, Tolmar, Vertex, and Viatris. He provided expert testimony for Janssen and Otsuka. He served on a Data Safety Monitoring Board for Compass Pathways, Denovo, Lundbeck, Relmada, Reviva, Rovi, Supernus, and Teva. He has received grant support from Janssen and Takeda. He received royalties from UpToDate and is also a stock option holder of Cardio Diagnostics, Kuleon Biosciences, LB Pharma, Mindpax, and Quantic.

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PLACEBO EFFECTS IN RANDOMIZED TRIALS OF PHARMACOLOGICAL AND NEUROSTIMULATION INTERVENTIONS FOR MENTAL DISORDERS: AN UMBRELLA REVIEW SUPPLEMENTARY APPENDIX

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Huneke, N.T.M., Amin, J., Baldwin, D.S. et al. Placebo effects in randomized trials of pharmacological and neurostimulation interventions for mental disorders: An umbrella review. Mol Psychiatry (2024). https://doi.org/10.1038/s41380-024-02638-x

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Efficacy and safety of omega-3 fatty acids supplementation for anxiety symptoms: a systematic review and dose-response meta-analysis of randomized controlled trials

• Negar Bafkar 1 ,
• Sheida Zeraattalab-Motlagh 2 ,
• Ahmad Jayedi 3 &
• Sakineh Shab-Bidar   ORCID: orcid.org/0000-0002-0167-7174 1 , 4  

BMC Psychiatry volume  24 , Article number:  455 ( 2024 ) Cite this article

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Background/Objectives

There is uncertainty about the optimum dose of omega-3 fatty acids for anxiety symptoms. We aimed to find the dose-dependent effect of omega-3 supplementation on anxiety symptoms.

We systematically reviewed PubMed, Scopus, and Web of Science until December 2022 to find randomized trials that assessed the effects of omega-3 fatty acids supplementation on anxiety symptoms in adults. Investigators performed the literature search and screened the titles/abstracts and full-texts and between-reviewer agreement was assessed as Cohen’s kappa coefficient. We conducted a random-effects dose-response meta-analysis to estimate standardized mean differences (SMD) and 95% confidence intervals (CIs) and assessed the certainty of evidence using the GRADE framework.

A total of 23 trials with 2189 participants were included. Each 1 gram per day supplementation with omega-3 fatty acids resulted in a moderate decrease in anxiety symptoms (SMD: -0.70, 95%CI: -1.17, -0.22; GRADE = low). The non-linear dose-response analysis indicated the greatest improvement at 2 g/d (SMD: -0.93, 95%CI: -1.85, -0.01), and that supplementation in a dose lower than 2 g/d did not affect anxiety symptoms. Omega-3 fatty acids did not increase adverse events (odds ratio: 1.20, 95%CI: 0.89, 1.61; GRADE = moderate).

Conclusions

The present dose-response meta-analysis suggested evidence of very low certainty that supplementation with omega-3 fatty acids may significantly improve anxiety symptoms, with the greatest improvements at 2 g/d. More trials with better methodological quality are needed to reach more robust evidence.

Protocol registration

PROSPERO (CRD42022309636).

Peer Review reports

Introduction

Anxiety is a psychological state that arises from excessive or disproportionate fear, and it is the most common psychiatric symptom that can cause distress or impairment [ 1 ]. Anxiety disorders are the leading mental disorders in the world [ 2 ]. An increase in anxiety symptoms, whether they’re emotional (like fear or apprehension) or physiological (such as a fast heart rate or trembling), is a shared characteristic among these disorders [ 3 ]. However, the diagnostic criteria for anxiety disorders vary greatly, including factors like how often and how severe the symptoms are, as well as whether the triggers for these symptoms are specific or broader [ 3 ]. In general, cognitive behavioral therapy is the most empirically supported psychological treatment for adults with anxiety disorders [ 4 ]. Drug therapies are also available for all anxiety disorders [ 4 ]; however, traditional medications, particularly at high doses or long-term usage, have some unfavorable adverse effects, which limit their utilization for the treatment of anxiety disorders [ 5 ]. Selective serotonin reuptake inhibitors (SSRIs) are effective in treating anxiety disorders. Furthermore, SNRIs (serotonin-norepinephrine reuptake inhibitors) influence outcomes more than a placebo does. The Food and Drug Administration (FDA) has approved venlafaxine, an SNRI, and the SSRIs paroxetine and sertraline. Benzodiazepines and the beta-blocker propranolol are also used to treat social anxiety disorder. Propranolol has the advantage of being used on an as-needed basis without the risk of developing dependence and tolerance, as exists with benzodiazepines [ 6 ].

Nutritional factors have a role in preventing and treating mental disorders [ 7 ]. Suboptimal nutrition has been implicated in the pathology of mental disorders and may impede treatment and recovery. Thus, nutritional interventions could potentially treat these disorders and are likely important for prevention [ 8 ].

Omega-3 polyunsaturated fatty acids, including α-linolenic acid (ALA) and docosapentaenoic acid (DPA), originate primarily from specific plant sources or are modified in plants, as well as including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) which are almost exclusively found in marine and algal sources [ 9 ]. Humans do not efficiently synthesize these fatty acids and need to consume them directly. Marine-derived omega-3 fatty acids (EPA and DHA) regulate dopaminergic and serotonergic neurotransmission and, thus, can affect anxiety symptoms [ 10 ]. The central nervous system has the highest concentration of these fatty acids in the human body after adipose tissue [ 11 ]. The brain needs sufficient and constant amounts of EPA and DHA for optimum function and a proper structure [ 12 ].

The possible mechanisms by which omega-3 related to anxiety were as follows. It is suggested that inflammatory responses are associated with anxiety [ 13 ]. Anxiety increases the production of pro-inflammatory cytokines such as interleukin-6 and tumor necrosis factor-alpha [ 14 ]. It has been indicated that the consumption of omega-3 reduces the production of pro-inflammatory cytokines [ 15 , 16 ]. Another possible mechanism is the expression of brain-derived neurotrophic factor (BDNF), which is a protein that can regulate the function of the nervous system [ 17 ]. When this protein is low, the synaptic growth of synergistic neurons in the brain is not stimulated, and its insufficient level is associated with depression and anxiety [ 18 , 19 ]. A review study including six studies with 469 participants showed that the consumption of omega-3 supplements could reduce anxiety symptoms through changes in four major mechanisms including inflammatory response, BDNF, cortisol, and cardiovascular activity [ 20 ].

Existing evidence on the efficacy of supplementation with omega-3 fatty acids in reducing anxiety symptoms is insufficient. An intervention trial revealed that omega-3 could improve anxiety among healthy subjects who encountered stressful evaluations [ 21 ]. A review study that evaluated the effects of omega-3 on anxiety indicated an improvement in anxiety symptoms (2.1 g/d EPA); however, the number of studies was very low ( n  = 1) [ 22 ]. A meta-analysis of intervention studies showed that supplementation with more than 2 g/day did have positive effects on anxiety symptoms [ 23 ]. Another review demonstrated that the consumption of omega-3 supplements, particularly through pathways related to inflammation, can lead to a decrease in anxiety symptoms [ 20 ]. However, two trials revealed no relation between omega-3 and anxiety disorders [ 24 , 25 ]. Moreover, the optimum dose of omega-3 fatty acids for reducing anxiety symptoms is still unclear. Therefore, in this systematic review of randomized controlled trials (RCTs), we intended to investigate the dose-dependent effects of omega-3 fatty acid supplementation on anxiety symptoms in adults.

We followed the guidelines from the Cochrane Handbook of Systematic Intervention Reviews [ 26 ] and the Grading Recommendations, Assessment, Development, and Evaluation (GRADE) handbook to carry out the present systematic study [ 27 ]. We registered our protocol for systematic reviews in PROSPERO (CRD42022309636).

Data sources and searches

We systematically searched three scientific databases, including PubMed, Scopus, and Web of Science, until February 2022, followed by an updated search to December 15, 2022. Working in duplicate, two investigators (NB and SZM) performed the literature search and screened the titles/abstracts and full-text articles in Endnote X9. The between-reviewer agreement was assessed and reported as Cohen’s kappa coefficient (κ) [ 28 ]. Disagreements were resolved by discussion with a third reviewer (SS-B). We also reviewed the reference list of meta-analysis studies of RCTs that investigated the effect of omega-3 on anxiety symptoms. Our search is limited to English-language articles. We described the complete search strategy in Table  1 .

Study selection

The inclusion criteria for the present review were determined according to the PICOS (population, intervention/exposure, comparator, outcome, as well as study design) approach and included the following items: (1) RCTs (study design), conducted on adults over 18 years of age, independent of drug usage or health status (population); (2) evaluation of the effect of oral omega-3 supplements including EPA, DHA, or ALA, in combination or individually and in various forms such as pills, oils, or fortified foods (intervention), compared to a control group (comparator); (3) considered a change in anxiety symptoms, assessed by formal diagnosis or an appropriate scale as continuous scale in participants with or without existing anxiety, as an outcome; (4) provided mean and standard deviation (SD) of anxiety symptoms at baseline and end of the study or reported sufficient information to estimate these values; and (5) provided dose of omega-3 supplementation in the intervention group. On the other hand, RCTs conducted on individuals under 18 years old, including pregnant or lactating women, were excluded.

Our primary outcome was a change in anxiety symptoms, while secondary outcomes were adverse events and health-related quality of life (HRQoL) and its components, including physical components, pain, general health, emotional well-being, and social functioning [ 29 ].

Data extraction

After the screening of the full texts, two investigators (NB and SZM) independently and in duplicate extracted the following characteristics from each trial: author’s last name, publication year, country, age range, baseline body mass index (BMI), sex, total sample size, duration of intervention, type intervention characteristics (dose of omega-3 supplementation in the intervention group), comparison group, calorie restriction, anxiety scale, baseline anxiety, any antidepressant drug usage, health status, outcome measures and main results for the outcomes included.

Risk of bias assessment

We evaluated the risk of bias using version 2.0 of the Cochrane tool for risk of bias [ 30 ]. Two authors (NB and SZM) independently evaluated the risk of bias in the trials, with disagreements resolved by the third author (AJ) when necessary.

Data synthesis and analysis

We considered the standardized mean difference (SMD) and its 95% confidence interval (CI) of changes in anxiety symptoms in the intervention group compared to the control group as the effect size for reporting the results of the present systematic review. First, we extracted the mean and SD of changes from baseline till the end of the intervention in each study arm in each trial. For those trials that did not report these changes, we calculated these values using the reported means and SDs of outcomes before and after the intervention using the Cochrane Handbook guidelines [ 26 ]. In the case of trials that reported standard errors instead of SDs, we converted them to SDs [ 31 ]. If SDs or standard errors were not reported in the trials, we used the mean SDs received from other trials for the analyses [ 32 ]. Second, for the analyses of continuous outcomes, we calculated SMD and its 95%CI for each 1 g/d increase in omega-3 fatty acids intake in each RCT using the approach introduced by Crippa and Orsini [ 33 ]. This method needs the number of participants in each study arm, dose of intervention, and the mean and SD of change across the study arm in each trial. Trial-specific mean and standard error of changes in outcomes for each 1 g/d increase in omega-3 fatty acids intake were pooled by applying the DerSimonian and Laird random-effects model [ 34 ]. We used SMD as an effect estimate because intervention trials used different questionnaires or scales (including the Barratt Impulsiveness Scale or Behavioral Inhibition/Behavioral Activation scale, etc.) to assess anxiety symptoms.

We performed predefined subgroup analyses according to baseline anxiety risk (high risk, specified as individuals with clinically diagnosed anxiety, using any diagnostic criteria; medium risk, specified as individuals with anxiety risk factors, such as long-term conditions; and low risk, specified as all other populations), intervention duration (≤ 12 versus ≥ 12 weeks) and risk of bias (high risk of bias, low risk of bias, some concerns). We selected baseline anxiety risk since it is an important effect modifier according to previous research. We also selected the risk of bias according to the GRADE instructions to determine whether the effects can also be seen in high-quality trials. Since adherence to the dietary interventions reduces over intervention duration, we selected intervention duration to determine whether the effects persisted in the long term. We also performed a meta-regression analysis to test the effect of intervention duration as a potential effect modifier.

Moreover, post-hoc subgroup analyses were according to the variables found in the literature search including supplement type (EPA, DHA, EPA + DHA), sex (men, women, both), weight status (normal weight, overweight/obese, not reported), health status (depressed, individuals with substance use, stressed, healthy, self-harm experience, ischemic stroke, Alzheimer disease, premenstrual syndrome), and antidepressant drugs usage (yes, no, mixed, not reported). According to eight criteria determined by the Instrument to assess the Credibility of Effect Modification Analyses (ICEMAN), we investigated the credibility of subgroup differences when the p -value for subgroup difference was < 0.10 [ 32 ]. ICEMAN consisted of 8 criteria to assess the credibility of the observed subgroup effects, one of which is the p -value for subgroup difference. According to the ICEMAN, when p for subgroup difference is 0.01–0.05, chance is a likely explanation, and when P  < 0.01, chance is an unlikely explanation. We followed their advanced approach to avoid over interpretation of subgroup effects (Supplementary Table 2 ).

We applied meta-regression analysis to calculate the p -value for subgroup differences. We examined the potential influence of any trial on the primary results by applying influence analysis and removing any RCT at once. We applied Egger’s [ 35 ] and Begg’s [ 36 ] tests for publication bias and examined asymmetry in the funnel plots. For assessing the heterogeneity across trials, we applied the I 2 statistic and conducted a χ2 test (P heterogeneity >0.10) [ 37 ]. Finally, we did a dose-response meta-analysis to clarify the dose-dependent effects of omega-3 fatty acids (g/d) on anxiety symptoms [ 38 ]. For the analyses of binary outcomes (adverse events), we computed the odds ratio and risk difference and their 95%CI using the number of participants and events in the intervention and control groups. STATA software version 17.0 was used for the analyses. A two-tailed p -value < 0.05 was regarded as statistically significant.

Grading of the evidence

We used the GRADE method to evaluate the certainty of evidence [ 39 ]. According to the GRADE approach, evidence obtained from RCTs is of high certainty, which can be downgraded or upgraded by predetermined criteria. To the interpretation of the magnitude of effect sizes, the estimated SMDs were interpreted as a trivial and unimportant effect (0.0-0.2), a small effect (0.2–0.6), a moderate effect (0.6–1.2), a large effect (1.2-2.0), a very large effect (2.0–4.0), and an extremely large effect (≥ 4.0) [ 40 , 41 ].

Systematic search

Figure  1 shows that the database and reference list searches identified 2215 records. After excluding 146 duplicates and an additional 2029 records through screening the title and abstract, 40 full texts were assessed for eligibility. Overall, 23 trials with 2,189 participants were eligible for inclusion in this dose-response meta-analysis [ 21 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 ]. The between-reviewer agreement for including studies was near perfect (Cohen’s kappa = 0.85) at the full-text screening step. The list of excluded full-text studies is shown in Supplementary Table 3 .

Literature search and study selection process

Characteristics of original trials

The general characteristics of the trials included in the present dose-response meta-analysis are described in Supplementary Table 4 . Eligible trials were published between 2007 and 2020. In terms of health status, the subjects in the seven trials were depressed [ 42 , 46 , 47 , 53 , 54 , 58 , 61 ], three trials included those with Parkinson’s disease [ 55 , 56 , 59 ], and one trial included participants with premenstrual syndrome [ 60 ], individuals with substance use [ 45 ], acute myocardial infarction [ 50 ], those with self-harm experience [ 51 ], stress [ 44 ], and stroke [ 57 ]. The other seven trials were conducted in healthy individuals. Of the 23 trials, eight were conducted in participants with normal weight [ 42 , 43 , 45 , 51 , 52 , 55 , 59 , 60 ], seven trials were conducted in those with overweight [ 44 , 45 , 48 , 53 , 55 , 57 , 62 ], and three in those with obesity [ 46 , 47 , 54 ]. Seven trials did not report the weight status of the participants in the study [ 21 , 49 , 50 , 56 , 58 , 61 , 63 ]. Nineteen out of the 23 trials had an intervention period of 12 weeks or less [ 21 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 57 , 58 , 60 , 61 ], and the other four trials had an intervention duration longer than 12 weeks [ 56 , 59 , 62 , 63 ]. Of the trials, 21 studies implemented a combination of EPA and DHA supplements [ 21 , 42 , 43 , 44 , 45 , 46 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 57 , 58 , 59 , 60 , 61 , 62 , 63 ], one study implemented DHA supplementation [ 56 ], and one trial implemented EPA supplementation [ 47 ]. Of 23 trials, seven trials had a low risk of bias (31, 33, 43, 45, 46, 49, 52), four had some concerns [ 49 , 53 , 57 , 63 ], and twelve were rated to have a high risk of bias (32, 34–37, 39–41, 44, 48, 50, 51) (Supplementary Table 5 ).

Primary outcome

Twenty-three trials with 1093 participants in the intervention group and 1096 in the control group reported information about the effect of omega-3 supplementation on anxiety symptoms [ 21 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 ]. Each 1 gram per day of omega-3 fatty acids resulted in a moderate decrease in anxiety symptoms (SMD: -0.70, 95% CI: -1.17, -0.22, p  < 0.001; I 2  = 97%; P heterogeneity < 0.001, GRADE = very low, Table  2 ) (Supplementary Fig.  1 ).

Table  3 indicates the subgroup analyses of the effects of omega-3 fatty acids (each 1 g/d) on anxiety symptoms. There was a significant subgroup difference by study risk of bias, where trials with a high risk of bias indicated a large and significant effect, but those with a low risk of bias did not indicate significance, as well as the magnitude of the findings. There were also other significant subgroup differences by weight status, medication use, and baseline risk of anxiety; however, in those cases, chance was a likely explanation (p subgroup difference between 0.01 and 0.05), and the credibility of subgroup difference was rated low (Supplementary Table 2 ) [ 32 ]. We did not find a significant or credible difference by intervention duration which was confirmed by meta-regression analysis (SMD per one-week increase: -0.01, 95%CI: -0.31, 0.06; P  = 0.54).

We observed no indication of publication bias with Egger’s test ( P  = 0.61), Begg’s test ( P  = 0.13), or with the inspection of the funnel plot (Supplementary Fig.  2 ). The dose-dependent effects of omega-3 on anxiety symptoms are shown in Table  4 ; Fig.  2 . The non-linear dose-response analysis indicated the greatest improvement at 2 g/d, where we found a moderate improvement in anxiety symptoms (SMD 2g/d : -0.93; 95%CI: -1.85, -0.01) (P dose−response : 0.051, P nonlinearity = 0.464; n  = 23, Fig.  2 ).

Dose-dependent effect of omega-3 fatty acids on anxiety symptoms. Solid lines represent standardized mean difference and dashed lines represent 95% confidence interval

Secondary outcomes

The effect of omega-3 fatty acids on secondary outcomes is reported in Table  2 . Omega-3 fatty acids did not increase adverse events. Supplementation with omega-3 fatty acids resulted in a small increase in social functioning (SMD: 0.52, 95% CI: 0.05, 0.98; GRADE = very low), but it did not increase other aspects of quality of life such as emotional well-being, general health, and physical component scale.

The certainty of evidence was rated very low for the effects of omega-3 fatty acids on anxiety symptoms. It was rated moderate for the effects of supplementation with omega-3 fatty acids on adverse events. The certainty of evidence was rated very low for other outcomes (Supplementary Table 6 ).

The present meta-analysis of intervention trials was the first study that addresses previous review limitations. It examines the dose-dependent effect of omega-3 fatty acids on anxiety symptoms, applying methodologies not previously utilized in similar studies. These methodologies include dose-dependent analysis to determine the optimal dosage for improving anxiety symptoms, utilization of ICEMAN for evaluating the credibility of subgroup differences, and application of the GRADE approach to assess the certainty of evidence in the included studies. We also conducted subgroup analysis according to baseline risk of anxiety, health status, and any antidepressant drug usage to identify which group benefits the most from omega-3 supplementation. Our findings showed that each 1 g/d omega-3 could moderately reduce anxiety symptoms. The dose-response meta-analysis suggested the greatest improvement at 2 g/d, and that higher doses of omega-3 supplements did not confer added health benefits.

A previous meta-analysis of 19 clinical trial articles ( n  = 2240 participants) demonstrated that omega-3 consumption in a dose below 2 g per day did not show any significant effects on anxiety symptoms. The participants included in the studies summarized in this meta-analysis were both healthy individuals and individuals with either a physical illness or mental disorder [ 23 ]. Another review study among individuals with no serious illness, including 31 trials, indicated that increasing omega-3 intake (300–3360 mg/d) might have little or no effect on reducing anxiety symptoms [ 64 ]. A reason for the contradictory results may be that in most original studies, the dose of omega-3 consumed was less than 2 g. Our dose-response meta-analysis indicated that supplementation with omega-3 fatty acids at a dose lower than 2 g/d did not significantly reduce anxiety symptoms.

Moreover, the subgroup analysis failed to show a significant and credible subgroup difference by intervention duration, and only four trials had an intervention duration longer than 12 weeks. In addition, we found a significant subgroup difference by study risk of bias, where trials with a low risk of bias did not show a significant effect. Therefore, more trials with better methodological quality and longer intervention duration are needed in this field.

We did not find a significant subgroup difference by supplement type (EPA versus DHA versus combined). This might be because the number of included studies for EPA and DHA supplements was very low (for EPA ( n  = 1) and DHA ( n  = 1); as a result, we were unable to find the difference between EPA and DHA. An intervention trial study indicated a significant decrease in anxiety and angry symptoms by daily intake of EPA & DHA (2,250 mg/d of EPA + 500 mg/d of DHA) for 12 weeks. They also indicated that both EPA and DHA serum levels were incremented; however, the EPA was more effective in improving anxiety symptoms and DHA was more effective in improving anger symptoms. This might be because of the different modes of action of long-chain omega-3 fatty acids for anger and anxiety [ 45 ]. Administration of EPA could reduce anxiety behavior in rats, as well as the stimulation of corticosterone by interleukin-1 beta [ 65 ]. Besides, changes in the way serotonin (5-HT) neurotransmission works in the brain can contribute to violent behavior, and consuming more DHA could help increase 5-HT neurotransmission in the brain. A study has shown that people with higher levels of plasma DHA were more likely to have higher levels of cerebrospinal fluid 5-Hydroxyindole Acetic Acid for both healthy individuals and late-onset alcoholics [ 66 ].

A cross-sectional study of 935 Australian adults indicated that those in the upper quartile of DHA intake had half of anxiety disorders compared to those in the lower quartile of DHA intake. No significant relationship was found for other types of omega-3 fatty acids, such as EPA [ 67 ]. However, of the 23 trials included in the present review, 21 used a combination of EPA and DHA, and thus, more trials are needed to determine whether EPA or DHA are superior to each other in reducing anxiety symptoms.

Regarding any antidepressant drug usage, we found a significant subgroup difference, where supplementation with omega-3 could significantly reduce the risk of anxiety symptoms in individuals who used any antidepressant drugs compared to individuals who did not use any antidepressant drugs or in combination. These findings indicate that omega-3 fatty acids might be more effective in individuals who use antidepressant drugs. A research study has shown that incorporating omega-3 as an add-on therapy has significantly enhanced the clinical effectiveness of antidepressant drugs such as sertraline. For example, combining antidepressant drugs with dietary and physiological supplements has amplified their antidepressant effects [ 68 , 69 ].

Our dose-response meta-analysis suggested that supplementation with omega-3 fatty acids at a dose of lower than 2 g/d had no effects on anxiety symptoms in adults. The greatest impact was also seen at this dose. This was consistent with a previous pairwise meta-analysis of intervention studies that suggested that the effects of omega-3 fatty acids on anxiety symptoms were stronger in the subgroup of trials with higher doses (at least 2 g/d) [ 23 ]. Our results provided additional practical information, suggesting that supplementation at a dose higher than 2 g/d did not confer additional decrement in anxiety symptoms.

Strengths and limitations

Our review had several strengths. First, this review was the first study to examine the dose-dependent effect of omega-3 fatty acids on anxiety symptoms. Second, we rated the certainty of evidence using the GRADE approach and utilized the MCID thresholds to determine whether the results were clinically important. Lastly, we conducted a subgroup analysis to find the source of heterogeneity and used the recently released ICEMAN tool for subgroup analyses. Among the limitations of our study, the variety of assessment methods for anxiety symptoms may also limit clinical interpretation and generalizability of the results. Moreover, we did not evaluate the potential effect modification by baseline omega-3 status in the study participants, which may affect our results. Examining this issue can help determine if supplementation is only required for deficient individuals or if it’s effective in all individuals, including those with normal intake. Also, we included adults regardless of their anxiety and health status. The results of our subgroup analysis also showed no significant effects in the subgroup of individuals with depression and high risk (baseline anxiety risk). Therefore, researchers should be careful about the interpretation of the results and discuss more carefully about these findings. Considering these limitations, more trials should be done on the possible effects of omega-3 on anxiety in individuals with anxiety and depression. Finally, we included only four studies with long-term duration, and thus, we could not thoroughly investigate the long-term effect of omega-3 fatty acids on anxiety symptoms.

In this systematic review and meta-analysis, it was seen that the consumption of each 1 gram of omega-3 per day reduced anxiety symptoms, but certainty of evidence was rated low. Dose-dependent analysis revealed that the maximum reduction in anxiety symptoms was seen in a dose of 2 g/d. However, further controlled trials with long-term follow-up and considering the baseline omega-3 status of the participants, as well as possible effects of omega-3 on individuals with anxiety and depression are needed to indicate more accurate results.

Data availability

The data sets generated or analyzed during the current study are not publicly available but are available from the corresponding author at reasonable request.

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Bafkar, N., Zeraattalab-Motlagh, S., Jayedi, A. et al. Efficacy and safety of omega-3 fatty acids supplementation for anxiety symptoms: a systematic review and dose-response meta-analysis of randomized controlled trials. BMC Psychiatry 24 , 455 (2024). https://doi.org/10.1186/s12888-024-05881-2

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Classification of recombinant factor VIII products and implications for clinical practice: A systematic literature review

Affiliations.

• 1 Clinical Division of Haematology and Haemostaseology, Department of Medicine I, Medical University of Vienna, Vienna, Austria.
• 2 PROMISE Department, University of Palermo, Palermo, Italy.
• 3 Haemophilia Treatment Center, Simone Veil Hospital, GH Eaubonne-Montmorency, Eaubonne, France.
• 4 CSL Behring, Milan, Italy.
• 5 CSL Behring, Paris, France.
• 6 CSL Behring, Marburg, Germany.
• 7 CSL Behring, Brussels, Belgium.
• 8 Institute of Experimental Hematology and Transfusion Medicine, University Hospital Bonn, Medical Faculty, University of Bonn, Bonn, Germany.
• PMID: 38549463
• DOI: 10.1111/hae.15001

Introduction: Consensus over the definition of recombinant factor VIII (rFVIII) product classification in haemophilia A is lacking. rFVIII products are often classified as standard half-life (SHL) or extended half-life (EHL); despite this, no universally accepted definition currently exists. One proposed definition includes half-life, area under the curve, and technology designed to extend half-life; however, the International Society on Thrombosis and Haemostasis defines activity over time as the most intuitive information for building treatment regimens and the World Federation of Hemophilia describes rFVIII product classification in terms of infusion frequency.

Aim: To summarise published data on the clinical and pharmacokinetic criteria used to define rFVIII product classification.

Methods: PubMed and EMBASE database searches of English-language articles (2002-2022) were conducted using search strings to identify the relevant population, intervention, and outcomes (e.g., clinical and pharmacokinetic parameters). Articles then underwent title/abstract and full-text screens.

Results: Among 1147 identified articles, 62 were included. Half-life was the most widely reported outcome with no clear trends or product groupings observed. No clear groupings emerged among other outcomes, including infusion frequency, consumption, and efficacy. As activity over time was reported in few articles, further investigation of its relevance to rFVIII product classification is warranted.

Conclusion: The findings of this systematic literature review suggest that parameters other than half-life might be important for the development of a comprehensive and clinically relevant rFVIII product classification definition. There seems to be an opportunity to consider parameters that are clinically meaningful and useful for shared decision-making in haemophilia A treatment.

Keywords: PubMed; factor VIII; haemophilia A; half‐life; pharmacokinetics; review.

© 2024 The Authors. Haemophilia published by John Wiley & Sons Ltd.

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