Introduction

Intergenerational trauma is the process by which the psychological impact of a traumatic event is transmitted from one generation to the next [1–4]. This results in subsequent generations experiencing the psychological effects of a traumatic event without exposure to it, such as increased risk of developing trauma- and stressor-related disorders [1–4]. Epidemiological studies provide some evidence for intergenerational trauma. Many studies to date have focused on the descendants of Holocaust survivors [5], whereas smaller bodies of literature have included other populations and traumatic events [1–4, 6, 7], focusing primarily on the transmission of trauma-related symptoms from the first (G1) to the second (G2) generation.

Researchers have evaluated intergenerational trauma through two main paths: (1) the path from parental trauma exposure to child psychopathology; and (2) the path from parental trauma-related symptoms to child psychopathology. One recent meta-analysis showed that being a child of a Holocaust survivor was modestly associated with increased posttraumatic stress disorder (PTSD) symptoms [8] and another meta-analysis of studies across any population or trauma-type identified a significant association between parental PTSD symptoms and child PTSD symptoms [9]. However, evidence is mixed, as additional reviews have been less conclusive [10, 11]. Researchers also speculate transmission from G1 to the third (G3) generation, which is supported by animal models [12, 13]. However, few epidemiological studies have evaluated transmission from G1 to G3 and the few that have obtained mixed results [14].

In addition to empirically demonstrating intergenerational trauma, it would be helpful to identify the causal processes underpinning this phenomenon as such findings could inform interventions aimed to prevent or mitigate it. Mediation studies examine the extent to which an intermediate variable (mediator) explains the effect of exposure on an outcome in an attempt to tease apart causal mechanisms [15]. Mediation studies quantify both direct and indirect effects, meaning the effect of exposure on the outcome and the effect of the exposure on the outcome through a mediating variable [15]. Epidemiological studies can also identify potential moderators, or variables that influence the degree of association between an exposure and an outcome. These relationships are often demonstrated visually through causal diagrams, most formally directed acyclic graphs (DAGs). DAGs can powerfully deduce causal relationships as they incorporate statistical approaches into a visual diagram [16]. Fig 1 demonstrates simplified DAGs for the intergenerational transmission of trauma including mediation (Fig 1A) and moderation (Fig 1B) paths.

Evidence suggests several complex, multifaceted causal processes underpin the intergenerational transmission of trauma. Although the most notable evidence is for genetic and epigenetic mechanisms, these do not fully account for transmission of trauma, suggesting the involvement of other psychosocial and interpersonal factors [17, 18]. Recent studies have provided some evidence that psychosocial factors that operate within the household play some role in mediating the intergenerational transmission of trauma [1, 3, 4, 17], with the strongest evidence supporting parenting behaviors, parental mental health symptoms, and attachment [1, 3, 4].

Understanding the role of psychosocial factors within the family in mediating and moderating the intergenerational transmission of trauma is critical to develop household-level psychosocial interventions that prevent transmission among high-risk families. Some psychosocial interventions have been explicitly developed to prevent the intergenerational transmission of trauma or have been evaluated for this purpose [19, 20]; however, these interventions are heterogeneous in their theoretical and therapeutic approaches and in the evidence used to their inform development. Understanding mediating and moderating factors could also optimize existing interventions, helping researchers understand what intervention component(s) are most effective and in what context(s).

To assess the need for a systematic review on this topic [21], we conducted a preliminary search to identify relevant literature reviews (eTable1 in S1 File). We located 21 reviews, most of which were scoping reviews focused on narrow sub-populations and trauma-event types (eTable 2 in S1 File). Only two reviews conducted moderation meta-analyses: one restricted to mothers with childhood maltreatment, and the other only including studies published until 2011. Based on these findings: (1) no review has published a diagram to demonstrate and contextualize epidemiological causal relationships at the family-level in the intergenerational transmission of trauma; (2) no review has meta-synthesized family-level mediators; and (3) there is a need to perform an updated meta-analysis of family-level moderators across diverse trauma-types.

We aim to fill these gaps by systematically consolidating the family-level psychosocial mediators, their respective direct and indirect effects, and moderators of such effects, on the intergenerational transmission of trauma. We will focus on a broad scope of populations and trauma types to increase the sample size available for meta-synthesis and to understand general mechanisms. We will also include impacts on the third generation (G3). By using the consolidation and meta-synthesis of epidemiological data, we will generate the first causal diagram to describe mechanisms through which trauma is transmitted intergenerationally.

Review questions

1. What family-level psychosocial mediator(s) and family-level psychosocial and demographic moderator(s) have epidemiological evidence supporting their involvement in the intergenerational transmission of trauma?

2. What are the combined quantitative effects across mediators and moderators in the intergenerational transmission of trauma?

Materials and methods

This project will follow review guidelines from the Joanna Briggs Institute (JBI) Manual for Evidence Synthesis [21] and the Cochrane Handbook for Systematic Reviews [22]. We followed the PRISMA-P [23] (eTable 3 in S1 File) reporting guideline to develop this protocol. This study has been registered on PROSPERO (registration ID #251053). We will post project materials, statistical code, and protocol amendments on our Open Science Framework webpage (https://osf.io/k5ezm/).

Inclusion criteria

We will use the systematic review PICOTS (Population, Intervention, Comparator, Outcome, Timeframe, Study design) framework to define eligibility, adapted to accommodate our research question focused on observational studies reporting meditator(s) and/or moderator(s) (Table 1). Definitions/elaborations documents will be available on Open Science Framework (https://osf.io/k5ezm/).

Population and exposure(s).

Included studies will have a population composed of at least one primary caregiver (G1) who experienced a traumatic event and their child (G2) and/or grandchild (G3) who did not experience this traumatic event firsthand. The G1 individual would not need to directly report their traumatic exposure, as long as there is some method to assess this, such as indirect reporting by G2. The traumatic event in G1 must have occurred before the birth of G2. G1 or G2 parents do not need to be biologically related to the child of interest. We will include trauma-exposed parents regardless of their trauma-related symptomatology. Animal studies will be excluded.

A traumatic event will be defined as exposure to threatened death, serious injury, or sexual violence, as outlined in the Diagnostic and Statistical Manual of Mental Disorders [24]. We will also consider parental trauma-related symptoms as the exposure, contingent that these symptoms are a direct result of an eligible traumatic exposure.

Mediator(s) and moderator(s).

Included studies will have at least one psychosocial factor operating at the family-level (including household- and parent-levels) that measured the interpersonal relationship between the child and their immediate family and/or primary caregiver before the child has reached 18 years of age. These could be framed or examined as mediators or moderators. Original articles would not need to explicitly use these terms or conceptualize factors in this way to be eligible.

Psychosocial factors might include parental psychiatric symptoms, suggesting that parental trauma-related mental health symptoms could be considered as the exposure or the mechanism, depending on the context. Factors might also include parental psychological factors, interpersonal parent-child relationships, or social factors within the household-level (which include demographic factors) that would influence child-rearing behaviors [25]. Comparators will be defined the measurement of the absence of the factor(s) or varying levels of continuously-measured factor(s) of interest in at least some sub-group of participants in the study sample. We will also include demographic factors (for example, age, gender, socioeconomic status, etc.) as moderators.

Outcome(s).

Included studies will quantify either the presence or absence of direct or indirect effects of trauma in the child (G2 or G3) under observation. Given the heterogeneity in clinical presentations of intergenerationally-transmitted trauma, we will only include quantitatively measured markers of psychological symptoms/functioning (i.e., child psychological development, psychopathology, and/or indicators of wellbeing). To increase our yield, we will be inclusive in what outcomes would qualify as child psychopathology and/or indicators of wellbeing for screening and may restrict this criterion after we assess the final sample of included studies. Outcomes will be considered in offspring. Psychiatric symptoms in the child would not need to be directly observed; for example, it would be acceptable for G1 to report on G2 symptoms.

Date and language restrictions.

This review will have no date restrictions, but will be limited to studies published in English, French, Kinyarwanda, and Spanish languages, for feasibility, as research team members are fluent in these languages.

Study design and document type.

We will include any epidemiologic study design, such as cross-sectional studies, case-control studies, cohort studies, randomized controlled trials, or the quantitative component of mixed-methods studies. Systematic review papers, meta-analyses, case-studies, qualitative studies, and study protocols will be excluded.

Due to feasibility, we will only include published literature (peer-reviewed literature and dissertations). One exception is that we will include conference abstracts during the screening phase contingent upon gaining access to the full text document (see Source of evidence selection). We included conference abstracts and dissertations to reduce publication bias.

Searches

The search strategy was developed and refined by a medical research librarian (KN) with the assistance of the first author. The final search strategy was developed using the following Boolean logic: [intergenerational concept] AND [family level concept] AND [transmission of trauma concept] using appropriate keywords and controlled vocabulary.

We will search MEDLINE, PsycINFO, PTSDpubs (formerly PILOTS), Scopus (conference papers only), and ProQuest Dissertations and Theses (doctoral dissertations only). eTables 4–8 in S1 File shows the final search strategy for each bibliographic database. Although we will only retrieve doctoral dissertations from Scopus, our PsycINFO search will capture dissertations of all levels. The final search was peer-reviewed by a second medical librarian using the Peer Review of Electronic Search Strategies (PRESS) checklist before the search was finalized [26].

We limited the search to five databases for feasibility given large preliminary search yields. However, to compensate for this limitation, we will implement three methods to increase sensitivity: (1) robust forward citation chaining of included studies; (2) reference list screening of relevant reviews from the authors’ personal libraries; and (3) screening additional papers from the authors’ personal libraries.

Source of evidence selection

Title/abstract and full-text screening will be performed independently by two reviewers using Covidence software [27]. Discrepancies will be resolved through screening from a third reviewer. During full-text screening, we will record the primary reason for exclusion.

We will screen the full-text versions of conference abstracts during full-text review. For each conference abstract, we will first attempt to locate full-text document online or by emailing the corresponding author. If we cannot obtain the full text using this process, we will exclude the record.

Data extraction

We will use Research Electronic Data Capture (REDCap) software to extract data from included studies [28]. Data extraction will be performed by one reviewer in combination with quality assurance checks. Discrepancies will be resolved through consensus, and when needed, discussion with a third reviewer. We will de-duplicate the results of individual studies reporting on the same underlying project and will use this combined information for our analyses. eTable 9 in S1 File shows the drafted data extraction items, which will likely be refined after piloting.

Training and piloting procedures

We will hold piloting and training procedures for reviewers before each screening stage. To prepare for title/abstract screening, reviewers will first review training materials, which will include a draft definitions/elaborations document. We will then undergo a series of training rounds to assess inter-rater agreement and identify discrepancies. In each round, reviewers will independently screen a random sample of 25 records from our final search yield to calculate percent agreement. Then, after each round, the team will discuss discrepancies and modify the definitions/elaborations document. We will repeat this process until we achieve team agreement of >75%, and will then begin title/abstract screening. We will follow similar procedures for full-text review training. Reviewers will first review training materials and draft definitions/elaborations document for full-text screening. We will then undergo a series of training rounds where each reviewer will independently screen a sample of approximately 10 records. Each round will be followed by discussion to modify screening procedures. We will begin full-text screening after we achieved agreement of >75%.

Prior to beginning data extraction, the first author will prepare and disseminate training materials for data extractors. Then, the team of data extractors will pilot data extraction and critical appraisal of 1–2 papers, discuss discrepancies, and repeat the process until achieving an agreement of >75% on each item before beginning extraction and appraisal.

Data charting

We will present summary information for all included studies in tabular format (eTable 10 in S1 File). We will then group the mediators presented in eTable 10 in S1 File together into similar thematic concepts, through discussion between the first and senior author. We will present these findings in a second table (eTable 11 in S1 File).

Meta-analyses

Our statistical analysis plan is presented below with the understanding that our analytical strategies may be refined based on the distribution of data collected and from input of statistical experts. Not all included studies will be eligible for meta-analysis. The first author in consultation from the senior author and project biostatistician (VS) will use information on the quality of reporting of results to decide what included studies will be eligible for meta-analyses. We will conduct meta-analyses using R statistical software and will consider a p-value < 0.05 as sufficient evidence to reject a null hypothesis.

Mediation meta-analyses.

We will use a mediation meta-analysis approach to pool data from individual mediation studies that report enough statistical information to derive indirect, direct, and/or total effects at the path-level. This could include reporting of beta coefficients directly (standardized or not standardized), or through back-calculating these estimates using correlation matrices, means, and variances/standard deviations for the exposure or traumatic exposure/symptoms (x), mediator of interest (m), and outcome or mental health status (y) variables. Our primary analyses will assess the following pathway: parental trauma exposure → psychosocial mechanisms (including parental trauma-related symptoms) → child mental health outcome. We will also assess the following pathway: parental trauma-related symptoms → psychosocial mechanisms → child mental health outcome. We will conduct several meta-analyses according to each mediating construct and whether it mediated G1 to G2, G2 to G3, or G1 to G3 effects. We will conduct sensitivity analyses to test whether we could consolidate findings of individual studies with different study designs and consolidate individual studies that measured the exposure as parental trauma versus parental post-traumatic symptoms. We plan to merge outcomes together to assess the global effect on child psychopathology; if there is enough data, we will sub-divide outcomes into different meta-analytic models. We will conduct meta-analyses following guidelines reported in the literature [29–31] that would be most suitable to for the methods reported in our sample of included studies, depending on the distributions and availability of statistical information reported in the included studies. We will aim to pool data as long as there are at least two beta estimates available for an eligible path.

Moderation meta-analyses.

We will aim to use a moderation meta-analysis approach to pool moderator data from individual studies, using similar methods as described above. Our primary analyses will assess moderating effects on parental trauma exposure → child mental health outcome; our secondary analyses will assess moderating effects on parental trauma-related symptoms → child mental health outcome. We will perform moderation meta-analyses using standard statistical methods [32], though we will take an atypical analytic approach given the nature of our research question. Traditional moderation meta-analyses assess the impact of study-level factors on the degree of association between an exposure and outcome [33, 34] and are restricted to assessing moderators that apply to every participant within analyses. For example, this could include moderating factors such as study design (such as whether cross-sectional studies versus cohort studies moderate the exposure-outcome relationship) or other population-level characteristics (such as study location or public versus private funding source). For the purpose of this review, however, it is unlikely this meta-analytic approach would be feasible to understand the moderating role of psychosocial family-level factors on the intergenerational transmission of trauma, as it is unlikely that individual studies would restrict their analyses to different levels for these types of factors, and thus, it would be unlikely that we would retrieve enough studies to assess differences in effect sizes using this traditional approach. For example, if we aimed to understand the moderating effect of parental communication style, it is unlikely that individual studies assessing the relationship between parental trauma exposure and child mental health outcome would have restricted their analyses to one specific level of communication style. To increase our likelihood of pooling estimates, and provide more meaningful narrative syntheses, we instead plan to combine individual studies that directly assessed for moderation as part of their analyses, as reported elsewhere [35]. We would aim to pool this data using an integrative data analysis approach (otherwise known as individual participant data meta-analysis) and would assess moderation using joint tests of interaction using linear regression models [33], though this will be contingent on securing original datasets from the included studies. Should it be challenging to obtain the individual-level datasets, we will report moderation findings using narrative syntheses.

Assessment of methodological quality.

We will assess the quality of mediation paths using the quality assessment tool used in other mediation meta-analyses [15]. To our knowledge, this is the only tool available to evaluate the quality of mediation studies. To assess the quality of moderation paths, we will assess the quality of included articles using the JBI critical appraisal tools, selected based on the study design of the included paper [36, 37]. We will apply this framework at the study level. We selected this tool as, to our knowledge, there are no tool developed specifically to evaluate the quality of moderation analyses. If we gather enough studies, we will conduct sensitivity analyses to assess whether study methodological quality impacts pooled estimates.

Publication bias

We aim to assess for publication bias using visual funnel plot inspection for each mediation meta-analysis conducted for each mediating construct. We do not plan to develop funnel plots for each moderating construct, as we do not anticipate having enough statistical information given that we anticipate only a small sample of included studies for each construct.

Presentation of results

Though we will aim to conduct meta-analyses wherever possible, in instances where this would not be possible (for example, not enough statistical information was reported, but the study provides quantitative information that would be qualitatively relevant to understanding causal pathways), we will report this information using narrative syntheses. We will also develop at least one causal diagram using data generated from pooled effect estimates. These diagram(s) will visually demonstrate the consolidated quantified relationships between variables and their relative magnitudes of association. We will include paths that are explicitly supported in our data and theoretical paths not explicitly supported (differentiated using solid versus dashed path lines). We will construct DAGs where possible if we obtain sufficient information, which would include combined effect estimates from our meta-analyses.

Assessing confidence in cumulative evidence

We will interpret our results using frameworks for causal inference [38] and/or Grading of Recommendations, Assessment, Development and Evaluations (GRADE) [39] to provide a narrative summary and recommendations for interventions.

Discussion

Strengths and limitations

Our methods are rigorous and follow the Cochrane Handbook for Systematic Reviews and Joanna Briggs Institute systematic review methods manual. We will take additional precautions to reduce likelihood of publication bias through forward citation chaining and including conference abstracts and dissertations. This review has several potential limitations. Although we consider our search strategy comprehensive, we will only search five databases and will not include Embase. We will not cover all languages, which might contribute language bias. We also might not locate enough studies to perform meta-analyses, and would instead produce narrative summaries of quantitative results. Our causal diagrams(s) will be restricted to psychosocial household-level factors and will exclude other relevant mechanisms outside of this scope such as biological factors. This review will also be unable to assess complex causal relationships between mediators, moderators, and their respective interactions; however, we will include this information in our narrative synthesis and causal diagram(s) if this information happens to be reported in individual studies.

Conclusions

The development of evidence-based causal diagram(s) would provide more detailed understanding of the paths by which the psychological impacts of trauma can be transmitted intergenerationally at the family level, including the relative strength of each factor in mediating and moderating cause and effect. These results could then be applied to design and optimize evidence-based interventions that target mechanisms with the strongest mediating effect. Taken together, this review could provide evidence to better support interventions that interrupt the cycle of intergenerational trauma for future generations.

Supporting information

Introduction

Intergenerational trauma describes the process where the psychological impact of a traumatic event passes from one generation to the next. This means later generations can experience psychological effects of trauma, such as a higher risk of stress-related disorders, even without direct exposure to the original event. Research in this area includes studies on the descendants of Holocaust survivors and other populations, mainly focusing on how trauma-related symptoms transfer from the first generation (G1) to the second (G2).

Researchers have explored intergenerational trauma through two primary avenues: the link between parental trauma exposure and child psychopathology, and the link between parental trauma-related symptoms and child psychopathology. Some studies have found a modest association between being a child of a Holocaust survivor and increased posttraumatic stress disorder (PTSD) symptoms. Other research across various populations and trauma types also noted a significant connection between parental PTSD symptoms and child PTSD symptoms. However, findings are not always consistent, and some reviews have been less conclusive. Transmission from G1 to the third generation (G3) is also hypothesized, supported by animal studies, but epidemiological evidence is mixed.

Beyond demonstrating intergenerational trauma, understanding its underlying causal processes is crucial. This knowledge could guide interventions designed to prevent or lessen its impact. Mediation studies investigate how an intermediate variable, or mediator, explains the effect of an exposure on an outcome, helping to reveal causal mechanisms. These studies quantify both direct effects (exposure on outcome) and indirect effects (exposure on outcome through a mediator). Epidemiological studies can also identify moderators, which are variables that influence the strength of the association between an exposure and an outcome. These relationships are often illustrated using causal diagrams.

Evidence points to complex causal processes behind the intergenerational transmission of trauma. While genetic and epigenetic mechanisms are well-supported, they do not fully explain transmission, suggesting that psychosocial and interpersonal factors also play a role. Recent studies indicate that psychosocial factors within the household, such as parenting behaviors, parental mental health symptoms, and attachment, are important mediators in the intergenerational transmission of trauma.

Understanding how family-level psychosocial factors mediate and moderate the intergenerational transmission of trauma is essential. This information can help develop household-level psychosocial interventions to prevent transmission in families at high risk. Some psychosocial interventions already exist or have been evaluated for this purpose, but they vary widely in their theoretical approaches and supporting evidence. Identifying mediating and moderating factors could also improve existing interventions by highlighting which components are most effective and in which situations.

Review questions

1. What family-level psychosocial mediators and family-level psychosocial and demographic moderators have epidemiological evidence supporting their involvement in the intergenerational transmission of trauma?

2. What are the combined quantitative effects across mediators and moderators in the intergenerational transmission of trauma?

Materials and methods

This project will follow review guidelines from the Joanna Briggs Institute (JBI) Manual for Evidence Synthesis and the Cochrane Handbook for Systematic Reviews. The PRISMA-P reporting guideline was followed to develop this protocol, and the study has been registered on PROSPERO.

Eligibility criteria for studies are defined using the systematic review PICOTS (Population, Intervention, Comparator, Outcome, Timeframe, Study design) framework, adapted for observational studies reporting mediators and/or moderators. The population includes at least one primary caregiver (G1) who experienced a traumatic event and their child (G2) or grandchild (G3) who did not directly experience that event. The G1 traumatic event must have occurred before the birth of G2. A traumatic event is defined as exposure to threatened death, serious injury, or sexual violence. Parental trauma-related symptoms are also considered an exposure if they result from an eligible traumatic event. Mediators and moderators include family-level psychosocial factors (e.g., parental psychiatric symptoms, parent-child relationships, household social factors) that affect the interpersonal relationship between the child and their immediate family or primary caregiver before the child reaches 18 years of age. Demographic factors are also considered moderators. Outcomes are quantitatively measured markers of psychological symptoms or functioning in the child (G2 or G3), such as child psychological development, psychopathology, or indicators of wellbeing. There are no date restrictions, but the review is limited to studies published in English, French, Kinyarwanda, and Spanish. Epidemiologic study designs, including cross-sectional, case-control, cohort studies, randomized controlled trials, or the quantitative component of mixed-methods studies, are included.

The search strategy was developed using Boolean logic with keywords for intergenerational trauma concepts and family-level concepts. Five databases will be searched: MEDLINE, PsycINFO, PTSDpubs, Scopus (for conference papers), and ProQuest Dissertations and Theses (for doctoral dissertations). To enhance sensitivity, the search will include robust forward citation chaining of included studies, screening of reference lists from relevant reviews, and screening additional papers from the authors’ personal libraries. Title/abstract and full-text screening will be performed independently by two reviewers using Covidence software, with a third reviewer resolving discrepancies. Data extraction will be performed by one reviewer with quality assurance checks using REDCap software. Training and piloting procedures will be conducted before each screening and data extraction stage to ensure high inter-rater agreement (above 75%).

Summary information for all included studies will be presented in tabular format. Mediators will be grouped into thematic concepts. Statistical analysis will involve meta-analyses using R statistical software. Mediation meta-analyses will pool data to assess pathways such as parental trauma exposure or parental trauma-related symptoms through psychosocial mechanisms to child mental health outcomes. Sensitivity analyses will test for consolidation of findings from different study designs or exposure definitions. Moderation meta-analyses will aim to pool moderator data, assessing moderating effects on the relationships between parental trauma exposure or symptoms and child mental health outcomes. If individual-level datasets can be obtained, an integrative data analysis approach will be used; otherwise, narrative syntheses will be provided. The methodological quality of mediation paths will be assessed using a specific quality assessment tool, while the quality of moderation paths will be evaluated using JBI critical appraisal tools. Publication bias will be assessed for mediation meta-analyses using funnel plot inspection.

Results will be presented through narrative syntheses where meta-analysis is not possible, and at least one causal diagram will be developed. These diagrams will visually demonstrate consolidated, quantified relationships between variables, showing both explicitly supported and theoretical paths. The findings will be interpreted using frameworks for causal inference and/or GRADE to provide a narrative summary and recommendations for interventions.

Discussion

This review employs rigorous methods that adhere to established systematic review guidelines, including the Cochrane Handbook and Joanna Briggs Institute manual. Additional measures are taken to reduce publication bias, such as forward citation chaining and the inclusion of conference abstracts and dissertations. However, several potential limitations exist. The search strategy will be limited to five databases and will not include Embase. Not covering all languages may introduce language bias. There is also a possibility that insufficient studies will be found to perform meta-analyses, which would necessitate narrative summaries of quantitative results. The causal diagram(s) developed will be restricted to psychosocial household-level factors, excluding other relevant mechanisms like biological factors. Furthermore, this review may not be able to assess complex causal relationships between mediators, moderators, and their interactions, although such information will be included in the narrative synthesis and causal diagram(s) if reported in individual studies.

Conclusions

The development of evidence-based causal diagram(s) will provide a more detailed understanding of how the psychological impacts of trauma can be transmitted across generations at the family level. This includes identifying the relative strength of each factor in mediating and moderating cause and effect. These results can then be applied to design and optimize evidence-based interventions that target mechanisms with the strongest mediating effect. Overall, this review has the potential to offer evidence that better supports interventions aimed at interrupting the cycle of intergenerational trauma for future generations.

Supporting information

Introduction

Intergenerational trauma describes how the psychological effects of a traumatic event can pass from one generation to the next. This means later generations may experience mental health impacts, such as a higher risk for trauma-related disorders, even if they were not directly exposed to the original event. Research on this topic has included studies on the descendants of Holocaust survivors and, to a lesser extent, other populations. Much of this work has focused on how trauma symptoms transfer from the first generation (G1) to the second (G2).

Researchers typically study intergenerational trauma in two ways: by looking at the link between parental trauma exposure and child mental health problems, or by examining the link between parental trauma symptoms and child mental health problems. Some meta-analyses have found a modest connection between being a child of a Holocaust survivor and experiencing post-traumatic stress disorder (PTSD) symptoms, and a broader link between parental PTSD symptoms and child PTSD symptoms across various populations. However, evidence is not always consistent, with some reviews showing mixed results. There is also speculation and some animal model support for transmission from G1 to the third generation (G3), but human studies on G1 to G3 transmission are limited and have yielded mixed findings.

Understanding the specific processes that cause intergenerational trauma is important because this knowledge can help develop strategies to prevent or reduce its effects. Mediation studies help identify intermediate variables that explain how an exposure leads to an outcome, quantifying both direct effects and indirect effects through a mediating variable. Epidemiological studies can also identify moderators, which are variables that change the strength of the relationship between an exposure and an outcome. These relationships are often illustrated using causal diagrams, such as directed acyclic graphs (DAGs), which visually represent causal connections.

Evidence indicates that several complex factors contribute to intergenerational trauma. While genetic and epigenetic mechanisms are significant, they do not fully explain the transmission, suggesting that other psychological and social factors within families also play a role. Recent studies have shown that psychosocial factors operating within the household, such as parenting behaviors, parental mental health symptoms, and attachment styles, contribute to mediating the transmission of trauma.

Identifying how family-level psychosocial factors mediate and moderate intergenerational trauma is crucial for creating effective interventions that can be implemented within families. Some psychosocial interventions have been developed or tested for this purpose, but they vary widely in their approaches and the evidence supporting them. A better understanding of mediating and moderating factors could also help refine existing interventions, highlighting which components are most effective and under what circumstances. The goal of this work is to systematically gather and combine epidemiological data on family-level psychosocial mediators and moderators of intergenerational trauma, including effects on G3, to create the first comprehensive causal diagram describing these transmission mechanisms.

Review questions

1. What family-level psychosocial mediators and family-level psychosocial and demographic moderators have epidemiological evidence supporting their role in the intergenerational transmission of trauma?

2. What are the combined quantitative effects of these mediators and moderators in the intergenerational transmission of trauma?

Materials and methods

This project will follow established guidelines for systematic reviews, including those from the Joanna Briggs Institute (JBI) Manual for Evidence Synthesis and the Cochrane Handbook for Systematic Reviews. The PRISMA-P reporting guideline was used to develop this protocol, which is registered on PROSPERO (registration ID #251053). All project materials, statistical code, and protocol amendments will be available on an Open Science Framework webpage.

Studies will be included if they involve at least one primary caregiver (G1) who experienced a traumatic event and their child (G2) or grandchild (G3) who did not directly experience that event. G1 trauma must have occurred before G2's birth, and biological relatedness is not required. A traumatic event is defined as exposure to threatened death, serious injury, or sexual violence, aligning with the Diagnostic and Statistical Manual of Mental Disorders. Parental trauma-related symptoms will also be considered an exposure if they result from an eligible traumatic event. Included studies must feature at least one family-level psychosocial factor, measured before the child reached age 18, that describes the interpersonal relationship between the child and their immediate family or primary caregiver. These factors might include parental psychiatric symptoms, parental psychological factors, parent-child relationships, or household social and demographic factors influencing child-rearing. Comparators will involve measuring the absence of these factors or varying levels of continuously measured factors. Outcomes must be quantitative measures of psychological symptoms or functioning in the child (G2 or G3), such as child psychological development, psychopathology, or wellbeing indicators. There are no date restrictions, but the review will be limited to studies published in English, French, Kinyarwanda, and Spanish. Any epidemiological study design will be included, such as cross-sectional, case-control, cohort studies, or quantitative components of mixed-methods studies. Systematic reviews, meta-analyses, case-studies, qualitative studies, and study protocols will be excluded. Only published literature (peer-reviewed articles and dissertations) will be included, with conference abstracts also considered during screening if the full text can be obtained.

The search strategy was developed with a medical research librarian, using keywords and controlled vocabulary to combine concepts related to intergenerational trauma, family-level factors, and trauma transmission. Searches will be conducted in MEDLINE, PsycINFO, PTSDpubs, Scopus (for conference papers), and ProQuest Dissertations and Theses. The search strategy was peer-reviewed using the PRESS checklist. To enhance sensitivity despite limiting the search to five databases, robust forward citation chaining of included studies, screening reference lists of relevant reviews, and reviewing additional papers from the authors' personal libraries will be implemented.

Title/abstract and full-text screening will be performed independently by two reviewers using Covidence software, with a third reviewer resolving discrepancies. Reasons for exclusion will be recorded during full-text screening. Efforts will be made to locate full-text documents for conference abstracts; abstracts without accessible full text will be excluded. Data will be extracted using REDCap software by one reviewer with quality assurance checks, and discrepancies will be resolved by consensus or with a third reviewer. Results from individual studies reporting on the same underlying project will be de-duplicated. Training and piloting procedures, involving multiple rounds of independent screening and discussion to achieve over 75% agreement, will be conducted for both title/abstract and full-text screening, and for data extraction and critical appraisal.

Summary information for all included studies will be presented in tabular format, and mediators will be grouped into thematic concepts. Statistical meta-analyses will be conducted using R statistical software, with a p-value less than 0.05 considered significant. Not all included studies will be eligible for meta-analysis, with eligibility determined by the quality of results reporting. Mediation meta-analyses will pool data from individual studies that report sufficient statistical information to derive indirect, direct, and/or total effects. Primary analyses will assess the pathway from parental trauma exposure through psychosocial mechanisms to child mental health outcomes, and secondary analyses will assess parental trauma-related symptoms through psychosocial mechanisms to child mental health outcomes. Multiple meta-analyses will be conducted based on each mediating construct and the generational transmission path (G1 to G2, G2 to G3, or G1 to G3). Sensitivity analyses will test the consolidation of findings across different study designs and exposure definitions. Outcomes will be merged to assess global effects on child psychopathology, or subdivided if enough data is available. Moderation meta-analyses will pool moderator data, assessing effects on parental trauma exposure to child mental health outcomes and parental trauma-related symptoms to child mental health outcomes. An integrative data analysis approach may be used if individual-level datasets can be obtained; otherwise, moderation findings will be reported through narrative syntheses.

The quality of mediation paths will be assessed using a specific mediation quality assessment tool. For moderation paths, the quality of included articles will be evaluated using JBI critical appraisal tools, selected based on study design. Sensitivity analyses will investigate whether study methodological quality affects pooled estimates. Publication bias will be assessed using visual funnel plot inspection for each mediation meta-analysis. For moderation, funnel plots are not anticipated due to an expected small sample size for each construct. When meta-analysis is not possible, quantitative information will be presented using narrative syntheses. At least one causal diagram will be developed using pooled effect estimates, visually demonstrating quantified relationships and their relative magnitudes. The diagram will include explicitly supported paths and theoretically relevant paths (differentiated by solid versus dashed lines). DAGs will be constructed if sufficient information is available. Results will be interpreted using frameworks for causal inference and/or GRADE to provide a narrative summary and recommendations for interventions.

Discussion

The methodology for this review is rigorous, adhering to established systematic review guidelines. Steps have been taken to reduce publication bias, such as forward citation chaining and including conference abstracts and dissertations. Potential limitations include the search being restricted to five databases and specific languages, which might introduce language bias. It is also possible that an insufficient number of studies will be found for meta-analyses, leading to narrative summaries of quantitative results instead. The causal diagram(s) developed will focus solely on psychosocial household-level factors, thereby excluding other relevant mechanisms like biological factors. Furthermore, this review will not be able to fully assess complex causal relationships involving multiple mediators, moderators, and their interactions, although such information will be included in narrative syntheses and causal diagrams if reported in individual studies.

Conclusions

Developing evidence-based causal diagram(s) will offer a more detailed understanding of the paths through which the psychological effects of trauma are transmitted between generations at the family level. This includes identifying the relative strength of each factor in mediating and moderating cause and effect. These findings can then inform the design and optimization of evidence-based interventions that target the mechanisms with the strongest mediating influence. Ultimately, this review aims to provide evidence to better support interventions designed to interrupt the cycle of intergenerational trauma for future generations.

Supporting information

Introduction

Intergenerational trauma describes how the psychological effects of a difficult event can be passed from one generation to the next. This means later generations might experience the mental impacts of a traumatic event without ever having lived through it themselves, such as having a higher chance of developing stress or trauma-related problems. Studies of populations show some proof of this. Many early studies focused on the children of Holocaust survivors, while a smaller number included other groups and traumatic events. These studies mainly looked at how trauma symptoms were passed from the first generation (G1) to the second generation (G2).

Researchers have mainly studied intergenerational trauma in two ways: first, by looking at how parental exposure to trauma affects a child's mental health; and second, by examining how a parent's trauma-related symptoms affect a child's mental health. One recent large-scale analysis found that being a child of a Holocaust survivor was linked to a slight increase in post-traumatic stress disorder (PTSD) symptoms. Another large-scale analysis across different populations and trauma types found a clear link between parents' PTSD symptoms and their children's PTSD symptoms. However, the evidence is mixed, and other reviews have been less certain. Researchers also believe trauma might pass from the first generation (G1) to the third generation (G3), a theory supported by animal studies. Still, few population studies have looked at G1 to G3 transmission, and those that have show mixed results.

Beyond simply showing that intergenerational trauma exists, it would be helpful to identify the reasons behind this phenomenon. Such findings could guide efforts to prevent or reduce its impact. Studies that look at "mediation" examine how much a middle step (a mediator) explains how an exposure affects an outcome. These studies measure both direct effects (the exposure's direct impact) and indirect effects (the exposure's impact through the mediator). Population studies can also find "moderators," which are factors that change how strong the link is between an exposure and an outcome. These relationships are often shown visually using diagrams called directed acyclic graphs (DAGs), which combine statistical methods with a visual display to understand cause-and-effect relationships.

Evidence suggests that several complex factors cause the intergenerational transmission of trauma. While genetic mechanisms and changes in gene expression (epigenetics) are the most noted, these do not fully explain how trauma is passed on. This suggests that other social and personal factors are also involved. Recent studies indicate that social and emotional factors within the household play a role in mediating the transmission of trauma. The strongest evidence supports the importance of parenting behaviors, parents' mental health symptoms, and attachment styles.

Understanding the role of family-level social and emotional factors in mediating and moderating the intergenerational transmission of trauma is crucial. This knowledge can help develop specific interventions within families to prevent trauma from being passed on to high-risk families. Some social and emotional interventions have been created or tested specifically to prevent this type of trauma transmission. However, these interventions vary greatly in their theories and treatment approaches, and in the evidence used to develop them. Understanding the mediating and moderating factors could also make existing interventions better by helping researchers understand which parts of an intervention are most effective and in which situations.

To determine if a full systematic review on this topic was needed, a preliminary search for relevant literature reviews was conducted. Twenty-one reviews were found, most of which were broad studies focusing on specific groups and trauma types. Only two reviews performed analyses of moderating factors: one was limited to mothers with childhood abuse, and the other only included studies published up to 2011. Based on these findings, it was determined that: (1) no review has published a diagram to show and explain the cause-and-effect relationships within families regarding intergenerational trauma; (2) no review has combined data on family-level mediating factors; and (3) there is a need for an updated analysis of family-level moderating factors across various types of trauma.

The goal is to fill these gaps by systematically bringing together family-level social and emotional mediating factors, their direct and indirect effects, and factors that moderate these effects, all related to the intergenerational transmission of trauma. The review will cover a wide range of populations and trauma types to gather more data for combining results and to understand general mechanisms. It will also include effects on the third generation (G3). By combining and analyzing population data, the review will create the first diagram that explains the mechanisms by which trauma is passed down through generations.

Review Questions

• What family-level social and emotional mediating factors, and family-level social, emotional, and demographic moderating factors, have scientific evidence supporting their role in the intergenerational transmission of trauma?

• What are the combined quantitative effects of these mediating and moderating factors in the intergenerational transmission of trauma?

Research Methods

The project will follow review guidelines from the Joanna Briggs Institute and the Cochrane Handbook for Systematic Reviews. This study has been registered on PROSPERO. Project materials, statistical code, and protocol changes will be shared on the Open Science Framework webpage.

Inclusion Criteria

Studies will be included if they feature at least one primary caregiver (G1) who experienced a traumatic event and their child (G2) or grandchild (G3) who did not directly experience that same event. The trauma must have happened to G1 before G2 was born. A traumatic event is defined as exposure to threatened death, serious injury, or sexual violence. Parental trauma-related symptoms will also be considered as an exposure.

Studies must include at least one social and emotional factor operating at the family level (household or parent level) measured before the child turned 18. These factors could be parental psychiatric symptoms, parent-child relationships, or social factors within the home.

The studies must also measure the presence or absence of direct or indirect effects of trauma in the child (G2 or G3). This includes quantitatively measured psychological symptoms, development, or wellbeing.

There will be no date limits, but studies must be published in English, French, Kinyarwanda, or Spanish. The review will include any epidemiological study design, such as cross-sectional, case-control, cohort studies, or the quantitative part of mixed-methods studies. Systematic reviews, meta-analyses, and qualitative studies will not be included. Only published literature (peer-reviewed articles and dissertations) will be considered, with conference abstracts included if full text can be obtained.

Search Strategy

The search strategy was developed by a medical research librarian and the first author. It used Boolean logic to combine terms for intergenerational concepts, family-level concepts, and trauma transmission. Databases like MEDLINE, PsycINFO, PTSDpubs, Scopus (for conference papers), and ProQuest Dissertations and Theses will be searched. The search strategy was reviewed by another medical librarian. To increase the number of relevant studies found, the team will also follow citations from included studies, screen reference lists of related reviews, and check additional papers from personal libraries.

Selecting Evidence

Two reviewers will independently screen titles, abstracts, and full texts using Covidence software. Differences will be settled by a third reviewer. The main reason for excluding a study will be recorded. For conference abstracts, efforts will be made to find the full text; if it cannot be obtained, the abstract will be excluded.

Data Collection

Data from included studies will be extracted using REDCap software. One reviewer will extract data, and quality assurance checks will be performed. Any differences will be resolved through discussion, involving a third reviewer if needed. Results from individual studies reporting on the same project will be combined for analysis.

Training and Testing Procedures

Before each screening stage, reviewers will receive training and participate in piloting procedures. For title/abstract screening, reviewers will first study training materials, then independently screen a random sample of 25 records. They will discuss differences and update guidelines until team agreement reaches over 75%. Similar steps will be followed for full-text review. Before data extraction, the first author will provide training materials. The data extraction team will then pilot data extraction on 1–2 papers, discuss differences, and repeat until achieving over 75% agreement before starting the main extraction and appraisal.

Organizing Data

Summary information for all included studies will be presented in tables. Mediators will then be grouped into similar themes through discussion between the first and senior author, and these findings will be presented in a second table.

Data Analysis and Meta-analyses

Meta-analyses will be conducted using R statistical software, with a p-value less than 0.05 considered significant. Not all included studies will be suitable for meta-analysis.

For mediation meta-analyses, data from individual studies will be combined if enough statistical information is available to calculate indirect, direct, or total effects. The main analysis will examine the path from parental trauma exposure to psychosocial mechanisms and then to child mental health. Another analysis will look at parental trauma-related symptoms to psychosocial mechanisms and then to child mental health. Separate meta-analyses will be done for each mediating factor and whether it affects G1 to G2, G2 to G3, or G1 to G3. Efforts will be made to combine different study designs and measures of exposure. Data will be pooled if at least two estimates are available for a path.

For moderation meta-analyses, a similar approach will be used to combine moderator data from individual studies. Primary analyses will assess how moderators affect the link between parental trauma exposure and child mental health, while secondary analyses will look at parental trauma-related symptoms. The aim is to combine findings from individual studies that directly assessed moderation. If obtaining original datasets is difficult, findings will be reported through narrative summaries.

The quality of mediation paths will be assessed using a specific quality assessment tool. For moderation paths, the quality of included articles will be evaluated using JBI critical appraisal tools, based on the study design. Sensitivity analyses will be performed if enough studies are gathered to see if study quality affects pooled results.

Checking for Bias

Publication bias will be assessed using visual funnel plot inspection for each mediation meta-analysis. Funnel plots are not planned for moderating factors due to an expected small sample of included studies for each.

Presenting Findings

If meta-analyses are not possible due to insufficient statistical information, quantitative data will be reported using narrative summaries. At least one causal diagram will be developed using combined effect estimates. These diagrams will visually show the relationships between variables and their strength. Diagrams will include both explicitly supported paths and theoretical paths, differentiated by line types. Directed acyclic graphs (DAGs) will be constructed if enough information is available.

Evaluating Evidence Confidence

Results will be interpreted using frameworks for understanding cause-and-effect and/or GRADE to provide a summary and recommendations for interventions.

Strengths and Limitations

The methods used are thorough and follow established systematic review manuals. Extra steps will be taken to reduce publication bias, such as tracking citations and including conference abstracts and dissertations.

This review has several potential limitations. Only five databases will be searched, and some languages will not be covered, which might lead to language bias. There might also not be enough studies to perform meta-analyses, in which case narrative summaries will be provided. The causal diagrams will focus only on family-level social and emotional factors, excluding other important factors like biological ones. The review will also not be able to fully assess complex relationships between factors, but this information will be included in narrative summaries and causal diagrams if available in individual studies.

Conclusions

Developing evidence-based causal diagrams will provide a more detailed understanding of how trauma's psychological effects can be passed down through generations at the family level. This includes identifying the relative strength of each factor in causing or influencing these effects.

The results of this review could then be used to design and improve interventions. These interventions would specifically target the mechanisms that have the strongest impact in passing down trauma.

Ultimately, this review aims to provide evidence that better supports interventions designed to interrupt the cycle of intergenerational trauma for future generations.

Supporting Information

Introduction

Trauma can affect people across generations, meaning the bad feelings from a hurtful event can pass from one generation to the next. This can cause children and grandchildren to experience problems like stress or trauma-related issues, even if they never went through the original event themselves.

Many studies have looked into this. A lot of early research focused on the families of Holocaust survivors. Later, other groups and types of traumatic events were studied. Most of this work focused on how trauma symptoms pass from parents (the first generation) to their children (the second generation).

Researchers have mainly looked at two ways trauma passes down:

1. When a parent's trauma leads to problems in their child's mental health.

2. When a parent's trauma symptoms lead to problems in their child's mental health.

Some studies found that children of Holocaust survivors had more symptoms of post-traumatic stress. Other studies also found a link between parents' trauma symptoms and their children's symptoms. However, not all studies agree, and the evidence is still mixed. There is also an idea that trauma might pass to a third generation, and some animal studies support this. But there are not many human studies on this, and their results are also mixed.

It is important to understand not just that trauma can be passed down, but also how it happens. Knowing the reasons could help create ways to stop or lessen the effects. Studies that look at "mediators" try to find middle steps that explain how one thing leads to another. "Moderators" are factors that change how strong a link is. These ideas can be shown in diagrams to help people understand them better.

Evidence suggests that many things work together to pass trauma down. While genes and changes to genes play a part, they do not explain everything. This means other things, like how families interact and what happens in daily life, are also important. Recent studies show that factors within the home, such as parenting styles, parents' mental health, and how children attach to their parents, play a role in this process.

Knowing about these family-level factors is key. It can help create special programs to prevent trauma from passing to the next generation in families at high risk. Some programs already exist to try and stop this, but they are all very different. Understanding the mediating and moderating factors can help make these programs better by showing what parts of an intervention work best and in what situations.

A search was done to see what research reviews already exist on this topic. Many reviews focused on very specific groups or types of trauma. Only a few looked at factors that change the strength of the link, and these were either very specific or outdated. This shows a need for a new review to:

1. Create a diagram showing how trauma passes within families.

2. Bring together all the information on family-level factors that mediate this process.

3. Provide an updated review of family-level factors that moderate this process across different types of trauma.

This study aims to fill these gaps by systematically gathering information on family-level factors that mediate and moderate the passing of trauma. It will look at many different groups and types of trauma to understand how it generally happens. It will also include effects on the third generation. By combining this information, the study will create the first diagram showing the causes and effects of how trauma is passed down in families.

Review Questions

1. What family-level factors, both social and related to demographics, have been shown by studies to be involved in how trauma passes from one generation to the next?

2. What are the combined measured effects of these factors in the passing of trauma from one generation to the next?

Materials and methods

This project will follow clear guidelines for research reviews. Information about the project, including how it will be done, has been registered publicly online.

Studies will be included based on specific rules. The studies must include at least one main caregiver (first generation) who went through a traumatic event, and their child (second generation) or grandchild (third generation) who did not experience that event firsthand. The caregiver's trauma must have happened before the child was born. The traumatic event is defined as being exposed to threatened death, serious injury, or sexual violence. Parents' trauma-related symptoms can also be considered the "exposure" if they resulted from a qualifying traumatic event. Studies will also look for at least one family-level factor (like things happening in the home or between parents and children) that played a part. These factors could include parents' mental health, their feelings, or how they interact with their children before the child turns 18. Outcomes in the child must be measured, showing their psychological health or problems. The review will include studies in English, French, Kinyarwanda, and Spanish, published at any time. Any type of study that looks at groups of people (like surveys or follow-up studies) will be included.

A librarian helped create a search plan to find relevant studies. The search will be done in five major research databases that hold information about health and psychology. To make sure as many studies as possible are found, the team will also look at the references of studies they find and check personal collections.

Two reviewers will independently check the titles and summaries of all found articles, then read the full articles to decide if they should be included. If they disagree, a third reviewer will help make the final decision. Information will be taken from the included studies using special software, and quality checks will be done. Reviewers will be trained before each step to ensure they follow the rules and agree on decisions.

Summary information from all studies will be put into tables. The main factors found will be grouped by similar ideas. Statistical methods will be used to combine data from many studies. This will help understand the overall effects of mediating factors (which explain how trauma is passed) and moderating factors (which change the strength of the trauma's impact). Studies will be checked for their quality and to see if the findings are likely to be real or biased.

The results will be presented in a way that is easy to understand. This will include writing summaries and creating one or more diagrams. These diagrams will visually show how different factors are linked and how strongly they affect the passing of trauma between generations. The team will also assess how confident they are in the overall evidence, and make suggestions for ways to help based on the findings.

Discussion

The methods used in this review are thorough and follow standard guidelines for research reviews. Steps are being taken to reduce the chance of bias, like including conference papers and doctoral studies, and looking at references of existing research. However, there are some possible limits. Only five databases will be searched, and not all languages will be included, which might leave out some important studies. It's also possible that there won't be enough studies to combine numbers, meaning the results would have to be described in words rather than by combining statistics. The diagrams will only show family-level factors and will not include other important things like biological factors. This review also will not be able to look at very complex ways that different factors interact, though this information will be mentioned if found in individual studies.

Conclusions

Creating diagrams based on strong evidence will help us better understand how the psychological effects of trauma pass between generations within families. It will show how strong each factor is in explaining or changing this process. These findings can then be used to design or improve programs that are meant to stop trauma from being passed down. In short, this review could offer important information to better support programs that help break the cycle of trauma for future generations.