5 Chapter 5: Literature Reviews

Learning Objectives:

• Understand the purpose and significance of a literature review in the research process.
• Plan a literature review by setting clear objectives and identifying key themes.
• Develop strategies for conducting comprehensive literature searches using various databases and tools.
• Gain skills in screening and selecting relevant sources based on predefined criteria.
• Master the organization of literature into coherent themes or chronological order to facilitate analysis.
• Acquire the ability to write a well-structured literature review using appropriate paragraph structuring techniques.
• Use proper citation to avoid plagiarism and give credit to original authors.
• Understand how to synthesize findings from the literature to inform research questions and hypotheses.
• Discover techniques for effectively revising the literature review and seeking constructive feedback.

Key Terms:

• Literature Review: A systematic evaluation of scientific articles that constitute the current state of knowledge in a particular research area, and in the context of a research study, the literature review justifies the new study.
• Thematic Organization: Grouping literature based on themes, theories, or methods to structure the literature review.
• Chronological Organization: Arranging literature in chronological order to demonstrate the evolution of research over time.
• Synthesis: The process of combining information from different sources to form a coherent understanding of the research area.
• Citation: The practice of giving credit to the original authors of the studies included in the literature review to avoid plagiarism.
• Inclusion/Exclusion Criteria: Predefined characteristics that determine which studies are relevant and should or should not be included in the literature review.

Introduction

We developed an intuition for literature reviews in Chapter 3 when discussing the importance of grounding research questions and hypotheses in scientific literature. This chapter covers the nuts and bolts of structuring a literature review. A literature review is the systematic evaluation of the scholarly and peer-reviewed articles that constitute the current state of scientific knowledge in a particular area. The literature review justifies the need for new research by identifying gaps in the current knowledge base and establishing the significance of the proposed research questions and hypotheses. To that end, the most important part of this chapter is Thematic Organization of Literature Reviews, which provides a list of guiding questions for researchers to use establish a justification for a research question that is grounded in the scientific literature.

The Purpose of a Literature Review

The purpose of a literature review in a research project is to justify the proposed research questions, hypotheses, and study designs. That is, researchers use the literature review in a project proposal or manuscript to justify why their particular research question or study design is necessary. Moreover, the literature review serves to clarify the research question or hypothesis, and establishes the context and rationale for the study within existing research.

The remainder of the chapter serves as a how-to guide for students writing literature reviews for project proposals or research reports.

Planning a Literature Review

Literature reviews were introduced earlier in discussions on formation of research questions and hypotheses in Chapter 3, in which we discussed grounding research questions and hypotheses in the scientific literature. From this step, you should have a general understanding of the literature within your area of investigation.

Based on a general intuition for the literature in a particular research area, the first step of planning a literature review often involves setting objectives for the literature review. Setting objectives for a literature review means defining the purpose of your literature review. Remember that the purpose of all literature reviews are to justify the proposed study, therefore, your literature review objectives are defined by the specific ways that you seek to use literature to justify your research proposal/project. By setting clear objectives, you can guide your literature search and ensure that you focus on the most relevant and important sources.

These are some example objectives for a literature review on social media and youth mental health:

• To explore the current state of knowledge on the relationship between social media use and mental health among adolescents.
• To identify the main theories, methods, and gaps in the existing research on social media use on adolescent mental health.
• To provide a conceptual framework for analyzing the effects of social media use on adolescent mental health.
• To inform the design of a survey-based study to investigate the causal impact of social media use on adolescent mental health.

Searching for Scholarly Sources

The next step is to find literature for your literature review. Google Scholar is a good place for general searches but below are additional search engines and strategies. Start by using a combination of keywords and phrases related to your research question. Keywords are specific terms or phrases that encapsulate the main concepts of your research topic. They are used to search for relevant articles in databases and search engines.

To enhance your search, consider using AI tools like ChatGPT. By providing ChatGPT with context about your research, it can generate a list of relevant keywords that you can use to refine your searches in platforms like Google Scholar, covering a broader range of relevant literature. Additionally, experiment with Boolean operators such as AND, OR, and NOT to refine your search, and consider using truncation (*) to include various word endings. To ensure a wide coverage of literature, search across multiple databases.

For example, when researching the effects of social media on adolescent mental health, you might start with a broad search query in Google Scholar like “social media adolescent mental health.” To refine your search, use Boolean operators and specific keywords, such as “social media AND anxiety AND adolescents.” This strategy helps narrow down the results to more relevant studies.

If you are working with the Adolescent Brain Cognitive Development (ABCD) Study, then you should review ABCD publications here. When you find studies that are relevant to your research topic then reviewing the references of those studies, which can lead you to additional literature. Apart from Google Scholar, other valuable tools include PubMed, a free search engine accessing primarily the MEDLINE database of references and abstracts on life sciences and biomedical topics, and Semantic Scholar, a free AI-powered research tool that helps discover scientific literature based on your search terms. Another innovative tool is Elicit, which allows you to ask research questions directly and provides a list of recommended papers without the need for converting questions into keywords.

Keeping track of search results and managing references is made easier with reference management software like Zotero. Zotero allows for easy organization, citation, and bibliography generation, helping you keep track of your sources efficiently.

Screening and Selecting Sources

After conducting a comprehensive literature search, the next step is to screen the most relevant sources for your literature review. This process involves evaluating the quality and relevance of the studies you’ve found to ensure that your review is based on reliable and pertinent information. Establish clear inclusion and exclusion criteria based on your research objectives, which might include factors such as the study’s publication date, research methodology, population studied, and relevance to your research question. For example, you might decide to include only peer-reviewed articles published in the last ten years that address your research topic from a particular methodological or theoretical perspective.

Start by reviewing the titles and abstracts of the studies to determine their relevance. Then, read the full text of the most promising studies to decide which ones to include in your review. Evaluate the quality of the studies by considering their research design, sample size, data analysis methods, and the credibility of the journal in which they were published. Assess the relevance of each study by determining how closely it aligns with your research question and objectives. This step helps to ensure that your literature review is built on a solid foundation of high-quality research.

For example, after conducting your search, you may have a list of 100 articles. To screen these, first, apply your inclusion criteria, such as “studies published in the last 10 years” and “studies focusing on social media’s impact on anxiety in adolescents.” Then, review the abstracts to exclude studies that don’t directly address your research question. This process might leave you with 30 articles for a more detailed review.

Lastly, depending on your research goals, it is likely important to verify whether an article is peer-reviewed. Not all content in Google Scholar (for example) is peer-reviewed, so it’s important to check the source of the article to confirm its credibility.

Organizing the Literature

After conducting a thorough search and screening process, it is time to organize the selected literature to begin to outline the literature review. As you save articles, maintain a running list of topics and subtopics within your research area. This list will serve as a guide for categorizing the literature and identifying key themes or trends.

When it comes to organizing the literature, we will focus on two common approaches: thematic and chronological organization (Patten 2016).

• Thematic Organization: This approach involves grouping literature based on themes, which are typically theoretical or methodological aspects of variables and/or their analysis. For example, if your research focuses on the impact of social media on adolescent mental health, you might organize the literature into themes such as: “Effects of Social Media on Self-Esteem,” “Social Media and Anxiety,” and “The Role of Parental Mediation.” Within each theme, you can compare and contrast the findings, methodologies, and theoretical frameworks of different studies, highlighting similarities and differences. Thematic organization facilitates the identification of patterns or gaps in the literature.
• Chronological Organization: Alternatively, you might choose to arrange the literature in chronological order, which can be particularly effective for showing the evolution of research over time. For instance, you could start with early studies on social media and adolescent mental health from the 2000s and progress to more recent research. This approach can help you trace the development of theories, changes in research focus, and advancements in methodology. It also enables you to demonstrate how understanding and perspectives on the topic have shifted over the years.

Regardless of the organization style you choose, you should summarize the key findings of the studies within each section and identify any gaps or inconsistencies in the literature. This will not only provide a comprehensive overview of the current state of research but also highlight areas where further investigation is needed, remember that your literature review serves to justify your study!

Thematic Organization of Literature Reviews

Thematic organization is arguably the most common in health research because it enhances the clarity and impact of the review by focusing on themes that directly relate to and highlight aspects of the research question and hypothesis. This method not only facilitates a deeper understanding of the topic but also helps in identifying and addressing gaps within the existing literature. Here are general steps to identify themes in the literature:

• 1) Identifying Themes: Begin by revisiting your research question and hypothesis to ensure alignment with the themes you choose to explore. Typical themes are directly connected to the study’s variables, and each theme is related to a variable or a relationship among variables.
• 2) Gathering and Sorting Literature: Use targeted searches with keywords that reflect each theme, such as searching your variables. Organize your sources by theme to facilitate an in-depth review of the literature in each specific area.
• 3) Synthesizing Information: For each theme, analyze the findings from your sources, noting major trends, methodologies, and results. Synthesize the information to draw overarching conclusions about the state of research in each theme. Highlight how these findings relate to your research question.
• 4) Highlighting Contradictions and Gaps: This is the most valuable step as disagreements, gaps, and limitations in the literature will justify your proposed research. These are some guiding questions to consider:
  • Contradictions in Findings
    • Can you identify and discuss any contradictions or conflicting findings among the studies? Any lack of consensus in the literature will serve your research purpose, as you can argue to provide additional insight on a debated research area.
    • How do the theoretical foundations of the studies compare? Often, contradictions in findings are rooted in different theoretical approaches. Highlighting these can underscore the importance of exploring a new or different theoretical framework in your research.
  • Identifying Gaps
    • Can you identify any neglected areas or aspects where the literature lacks depth? These gaps provide a strong justification for your research, underscoring the need for further investigation.
    • Was your research topic studied from a particular theoretical perspective or methodological approach that was narrow in scope? This will serve your argument for justifying further research on the topic. For example, if a topic was studied with only a particular demographic, then this would warrant your study on a more diverse population.
    • Are there new or emerging trends that have not been sufficiently studied? Identify trends that are gaining momentum but have not been extensively researched. Your work could provide early insights into these areas.
    • What are the limitations acknowledged by authors in the field? Review the limitations sections of key studies to find common themes about what further research is needed—these are direct invitations for new studies to build upon previous work.
  • Broadening Scope
    • Has prior research been limited by its scope or data collection methods? Propose how your methodology or broader scope could provide more comprehensive data or insights.
    • What are potential new applications of existing theories or data? Suggest how applying theories or data in new contexts or in interdisciplinary ways could fill existing knowledge gaps.
• 5) Structuring Your Findings: Make sure to directly connect how addressing these gaps, contradictions, or limitations will help answer your primary research question, thereby adding value to your field.

Evaluating Methodologies

In the process of organizing a thematic literature review, evaluating the methodologies used in the studies you work with is a common element of identifying gaps in the literature (to the point above). This evaluation involves several key steps:

• Identify Method Types: First, identify the type of methodology employed in each study. Research methodologies generally fall into three categories: qualitative, quantitative, or mixed methods. Qualitative methods are useful for gaining detailed insights into complex issues but may lack generalizability. Quantitative methods, on the other hand, provide measurable and often generalizable results but might not capture the depth of human experiences. Mixed methods combine both qualitative and quantitative approaches to leverage the strengths of each, providing a comprehensive understanding of the research problem.
• Assess Appropriateness: Next, evaluate whether the methodologies used are suitable for the research questions addressed. This involves considering whether the methods align with the objectives of the study and whether they are capable of effectively addressing the research questions posed. For instance, if a study aims to understand the emotional impacts of a medical condition, a purely quantitative approach might miss nuanced personal experiences that a qualitative method could capture.
• Consider Biases: It is also vital to discuss potential biases inherent in the methodologies. Every research method has its limitations and potential biases, which can affect the findings. For example, survey research can suffer from self-report bias, while observational studies might be influenced by the observer’s presence. Recognizing these biases helps in critically analyzing the studies’ findings and assessing their reliability and applicability to your research.
• Implications for Your Study: Finally, consider how these methodologies influence the reliability and validity of your own research. Understanding the strengths and weaknesses of different methods used in your field will guide you in choosing the most appropriate approach for your study. This step is about drawing lessons from the reviewed literature to enhance your research design, reduce potential biases, and improve the overall reliability of your findings.

Writing the Literature Review

Before writing the actual literature review, it’s essential to create an outline based on the themes identified in the “Organizing the Literature” section above. This way you can make changes while seeing the big picture of the literature review. This outline will serve as a roadmap for your literature review, ensuring that each section is well-organized and focused.

Creating an Outline

• Outlines take the form of bullet points like this, or can be numbers or roman numerals, and this will be the skeleton of your literature review, and we want our literature reviews to have a strong outline as this backbone.
• Begin by listing the main themes or topics that emerged from your organization of the literature. These will form the major sections or subtopics of your outline.
• For each theme, list the key studies or articles that you have identified as relevant. Summarize the main findings or arguments of each study, highlighting how they contribute to the theme.

At the end of this chapter are two examples of literature review outlines based on real ABCD studies.

Writing the Paragraphs

After outlining your literature review, you are ready to write it. In most published studies in health sciences, literature reviews can be found in the Introduction section of the article. This is because the literature review justifies the entire study, so it is the first information presented. However, in many instances, especially in social sciences, literature reviews may be found in a separate section of the article, which is also the case for many research proposals.

Below are two strategies for writing the literature review, one for beginners and intermediate students and the other for students ready for a more advanced challenge.

For those at a beginner or intermediate level, consider using the PIER system to structure your paragraphs:

• Point: Begin each paragraph with a clear topic sentence that introduces the main point or argument.
• Evidence: Support your point with evidence from the literature, such as data, findings, or quotes from relevant studies.
• Explanation: Elaborate on how the evidence supports your point and discuss its implications for your research topic.
• Repeating/Linking Sentence: Conclude the paragraph with a sentence that reinforces the main point and creates a smooth transition to the next paragraph or section.

For example, in a paragraph about the benefits of urban green spaces for mental health, you might start with a Point: “Research has consistently shown that urban green spaces contribute to mental well-being.” Then, provide Evidence: “A study by Thompson et al. (2012) found that individuals who spent time in urban parks reported lower stress levels.” For Explanation: “These findings support the biophilia hypothesis, suggesting that humans have an innate affinity for nature.” Finally, a Repeating/Linking Sentence: “Thus, the presence of green spaces in cities can be a crucial factor in promoting public health.”

The best literature reviews synthesize literature rather than summarizing it. For more advanced writers, synthesizing studies means integrating them conceptually and presenting them together in different categories of literature. Synthesis allows you to build a narrative that integrates various perspectives, while critical analysis helps to identify gaps and limitations in the existing research. Two examples of these are in the following section. These are general steps to synthesize literature:

• Gather Information: Compile results from all selected studies relevant to each theme.
• Compare Findings: Identify similarities and differences in results, methodologies, and conclusions.
• Integrate Results: Merge findings to form a unified view of the theme. Discuss how these findings relate to your hypothesis.
• Highlight Significance: Explain how the synthesis informs your research question and fills existing knowledge gaps.

Concluding the Literature Review

Conclude your literature review with a discussion how the literature informs your research question and underscores the necessity of your study. Highlight how your review contributes to the existing body of knowledge and how it lays the groundwork for your research. This is your opportunity to articulate the significance of your work and its potential impact on the field.

Citing Sources

Proper citation is essential to avoid plagiarism and give credit to the original authors. Make sure to use a consistent citation style throughout your review, such as APA, MLA, or Chicago, depending on the requirements of your academic field or institution.

Seeking Feedback

Feedback from peers and mentors can be invaluable in refining your literature review. Share your draft with colleagues or advisors and be open to their suggestions and critiques. Constructive feedback can help you identify areas for improvement and strengthen the overall quality of your review.

Example Literature Reviews with ABCD

In this section, we will consider examples of outlines of real literature reviews from real ABCD studies. The database of ABCD studies can be found here.

Example One

The first article we will consider is this:

Weissman, D.G., Hatzenbuehler, M.L., Cikara, M. et al. State-level macro-economic factors moderate the association of low income with brain structure and mental health in U.S. children. Nat Commun 14, 2085 (2023). https://doi.org/10.1038/s41467-023-37778-1

The literature review section of this study is in the Introduction section.

Research question: How does state-level macroeconomic factors, such as cost of living and anti-poverty programs, influence the association between low income and brain structure and mental health in children? (Weissman, Hatzenbuehler et al. 2023)

The hypothesis is that lower family income would be associated with smaller hippocampal volume and higher internalizing and externalizing problems, and that these associations would be moderated by state-level macrostructural characteristics.

Let’s reverse engineer an outline of the literature review of the study:

• Impact of Family Income on Childhood Development
  • Lower family income correlates with lower educational attainment, reliance on public assistance, and health issues in adulthood (Duncan, Ziol-Guest, & Kalil, 2010).
  • Socioeconomic status (SES) influences brain structure and mental health, affecting later-life outcomes (McLaughlin et al., 2011; Peverill et al., 2021).
• Socioeconomic Influences on Brain Development
  • Consistent findings show smaller hippocampal volumes in children from lower-income families (Hair et al., 2015; Hanson et al., 2013).
  • Economic hardship limits resources available for child development, increasing exposure to stress (Evans, 2004; Evans et al., 2005).
• Neurological Impacts of Stress
  • Chronic stress and adversity, common in low-income settings, detrimentally affect hippocampal structure (McEwen & Magarinos, 1997; Ivy et al., 2010).
• Role of Environmental and Macrostructural Factors
  • Differences in state-level economic policies and cost of living can exacerbate or mitigate the impacts of low income on brain development and health (Duncan et al., 2010).
• Influence of Public Policies
  • Anti-poverty programs like Medicaid and Temporary Assistance for Needy Families (TANF) may buffer negative effects of low SES on neurodevelopment (Costello et al., 2003; Noble et al., 2015).
  • Variability in policy implementation across states impacts the effectiveness of these interventions (Duncan et al., 2010).
• Comparison with European Contexts
  • Studies suggest weaker associations between income and brain volume in European countries, potentially due to different social policies (Walhovd et al., 2022).
• Hypotheses
  • Presents the study’s hypotheses that lower family income will be associated with smaller hippocampal volume and higher internalizing and externalizing problems, and that these associations will be moderated by state-level macrostructural characteristics.

The authors likely structured the literature review in this manner to build a comprehensive argument that socioeconomic factors influence brain development through various direct and indirect pathways, emphasizing the importance of their study within this context. This justifies the study’s focus on state-level macrostructural characteristics by showing that these factors can either exacerbate or buffer the effects of low SES on brain development. Such argumentation positions their research within a larger dialogue about how policy interventions could potentially address disparities in neurodevelopmental outcomes, making a case for the practical implications of their findings. By framing the literature review in this way, the authors effectively lay a solid foundation for their research questions and hypotheses.

Example Two

Next let’s consider a different published study from ABCD:

Pollmann A, Sasso R, Bates K, Fuhrmann D. Making Connections: Neurodevelopmental Changes in Brain Connectivity After Adverse Experiences in Early Adolescence. J Neurosci. 2024 Feb 21;44(8):e0991232023. doi: 10.1523/JNEUROSCI.0991-23.2023. PMID: 38124022; PMCID: PMC10883609.

Research question: How do adverse childhood experiences impact neurodevelopmental trajectories in terms of brain connectivity? (Pollmann, Sasso et al. 2024)

The hypothesis is: (1) White matter connectivity, measured in fractional anisotropy (FA), increases from ages 9 to 13 years. (2) There will be individual differences in trajectories of white matter connectivity, with subpopulations showing distinct structural brain connectivity trajectories. Alongside differences in global brain connectivity patterns, the authors expected differences in the corpus callosum, fornix, and anterior thalamic radiations based on previous findings. Additionally, the presence and severity of adverse childhood experiences (ACEs) will modulate these trajectories, with distinct connectivity patterns emerging in adolescents with different levels of exposure to ACEs.

An outline of the literature review is as follows:

• Prevalence and Impact of Adverse Childhood Experiences (ACEs)
  • Global prevalence of ACEs (Cicchetti, 2016; Carlson et al., 2020).
  • Link between youth adversity and various negative outcomes (Felitti et al., 1998; Dube et al., 2003; Hughes et al., 2020; Liao et al., 2021).
• Neurological Changes Due to Adversity
  • Role of neurological changes in developmental pathways affected by adversity (Short and Baram, 2019; Lim et al., 2020).
  • Impact of adversity on brain connectivity and mental health issues, particularly white matter integrity (Benedetti et al., 2014).
• Brain Connectivity and Developmental Outcomes
  • How brain regions are interconnected via white matter tracts affecting various phenotypes (Johansen-berg et al., 2010).
  • Influence of childhood maltreatment on microstructural development of white matter and increased vulnerability to mental health issues (De Bellis et al., 2002; Paul et al., 2008; Huang et al., 2012; Lu et al., 2013).
• Methods to Investigate Neurodevelopmental Changes
  • Use of MRI and diffusion tensor imaging (DTI) to explore brain microstructural properties (Sporns, 2013; Alexander et al., 2007; Beaulieu, 2002).
  • Fractional anisotropy (FA) as a measure of white matter integrity, affected by factors like myelin insulation or axonal injury (Fields, 2008; Thomason and Thompson, 2011).
• Adolescence as a Sensitive Period for Development
  • Description of adolescence as a period of significant social and biological changes, potentially making it a sensitive period for neurodevelopment (Andersen and Teicher, 2008; Kolb, 2009; Fuhrmann et al., 2015).
  • Differences between Adverse Adolescent Experiences (AAEs) and ACEs, highlighting the need to study neurodevelopment during adolescence specifically (Pollmann et al., 2022).
• Study Rationale and Hypotheses

The organizing logic of the literature review highlights the variables in the study, structured around the impact of adverse experiences on neurodevelopment, specifically focusing on brain connectivity during adolescence. It starts by establishing the widespread prevalence and significant impact of adverse childhood experiences (ACEs), then narrows down to the specific neurological changes, particularly in white matter integrity, as mechanisms through which adversity impacts long-term outcomes. This progression logically leads to explaining the use of MRI and DTI techniques, underscoring their relevance in measuring brain structure changes. Finally, it highlights adolescence as a critical period for brain development, influenced by external factors, emphasizing the necessity of focusing on this life stage to fill gaps in current research and potentially guide interventions. This structure not only educates but methodically builds up to the research question, justifying the study’s focus and methodological approach.

Summary

Crafting a comprehensive literature review not only showcases the depth of your understanding of the subject matter but also lays the groundwork for your study by highlighting its relevance and necessity. As we transition to the next chapters on sampling and instrumentation, it’s important to remember that the insights gained from the literature review will guide the selection of an appropriate sample, tools, and methods for data collection and analysis, ensuring that your research is well-equipped to address the questions and hypotheses that you’ve set out to explore.