Maltreated children might be more likely to develop mental health problems by virtue of their exposure to abuse and neglect. Alternatively, their risk might be higher because they have other genetic and environmental risk factors for mental health problems, which confound previously observed associations. For example, evidence suggests that maltreated children are likely to have family histories of mental illness (
9) and may have higher polygenic scores for psychiatric disorders (e.g., schizophrenia, depression, and ADHD) (
10–
12). Maltreated children are also likely to experience other environmental risks for mental health problems, such as socioeconomic disadvantage (
9) and bullying victimization (
13). However, these co-occurring risk factors have not been not fully accounted for in the majority of previous research on maltreatment and mental health, which used classical epidemiological methods (e.g., multiple regression) with limited ability to account for genetic influences or other unmeasured confounders.
Quasi-experimental methods offer a solution to help disentangle the causal effects of childhood maltreatment on mental health from confounding (
14). Four broad categories of quasi-experimental methods have been used to examine the relationship between maltreatment and mental health. First, family-based designs (e.g., twin or sibling differences, children of twins, and adoption designs [
15]) capitalize on varying genetic and environmental relationships between family members to examine the effects of maltreatment independent of familial confounding. Second, panel data designs (e.g., fixed-effects methods, random-intercept cross-lagged models) leverage longitudinal data to test whether within-individual changes in maltreatment exposure predict changes in mental health, independent of stable individual factors. Third, natural experiments examine the effects of maltreatment that is not related to family or individual risk factors, but rather occurs as a result of wider social or political processes. For example, the English and Romanian Adoptees (ERA) Study (
16) examined the mental health consequences of institutional neglect that occurred during the Ceaușescu regime in Romania, where widespread poverty and bans on contraception led to thousands of babies being abandoned in orphanages. Fourth, propensity score methods (e.g., propensity score matching; inverse probability weighting) aim to mimic a randomized experiment by statistically removing confounding by (measured) preexisting differences between maltreated and non-maltreated individuals. Although propensity score methods do not directly account for unmeasured confounding, they have been found to be more effective than multiple regression in reducing confounding (
17).
While a number of individual studies have applied these quasi-experimental methods to strengthen causal inference about the relationship between maltreatment and mental health, there has been no systematic summary of this evidence. Such a systematic evaluation is important to 1) estimate the pooled magnitude of a potentially causal relationship between maltreatment and mental health, 2) examine whether findings triangulate across quasi-experimental methods with different assumptions and sources of bias, and 3) examine whether findings differ according to key moderators, such as the type of mental health outcome, form of maltreatment, or assessment method. To address this research gap, we conducted a systematic review and meta-analysis of quasi-experimental studies on childhood maltreatment and mental health problems.
Discussion
To our knowledge, this is the first meta-analysis to examine the relationship between childhood maltreatment and mental health in quasi-experimental studies. Across 34 studies that included over 54,000 individuals, our meta-analysis provides novel insights into causality, confounding, and specificity of the relationship between childhood maltreatment and mental health.
Regarding causality, we found that childhood maltreatment had a small association with mental health problems (Cohen’s d=0.31) after stringent quasi-experimental control for confounding. Notably, this association triangulated across multiple types of quasi-experimental methods with different assumptions and potential sources of bias, strengthening causal inference (
58,
59). Although a number of included studies used retrospective measures of maltreatment, findings were consistent among studies using prospective measures, suggesting that recall bias does not fully explain the relationship between maltreatment and mental health. Furthermore, while some studies were cross-sectional, we also observed consistent results in longitudinal studies controlling for preexisting mental health problems, suggesting that the findings are not due to reverse causation. Taken together, this evidence is consistent with a small causal contribution of maltreatment to mental health. Although small, these effects of maltreatment could have far-reaching consequences, given that mental health problems predict poor outcomes in major life domains, such as occupational attainment (
60), physical health (
61), and mortality (
62).
Regarding confounding, we found that the association between childhood maltreatment and mental health in quasi-experimental adjusted models was substantially (45%) smaller than in unadjusted models. Of note, the unadjusted association was moderate in magnitude, with a Cohen’s d of 0.56, which is similar to effect sizes reported in previous meta-analyses of non-quasi-experimental studies (see Table S10 in the
online supplement). This reduction in effect size after quasi-experimental adjustment suggests that a large part of the overall relationship between childhood maltreatment and mental health is confounded by preexisting risk factors for psychopathology. Research is needed to identify the specific factors that elevate risk of psychopathology in maltreated children, which might include environmental adversities (e.g., socioeconomic disadvantage [
9]) and genetic liability (
10–
12).
Regarding specificity of effects, we observed three key findings. First, quasi-experimental evidence suggested small causal effects of maltreatment on a broad range of mental health outcomes, including internalizing disorders (e.g., depression, anxiety, suicidality, self-harm), externalizing disorders (e.g., conduct problems, ADHD, alcohol and drug abuse), and psychosis, rather than a few specific outcomes. This finding supports evidence from non-quasi-experimental studies (
63–
67) and suggests that maltreatment may affect broad factors underlying multiple disorders (e.g., altered brain structure and function [
68,
69], emotional dysregulation [
70], or information-processing biases [
71]) rather than disorder-specific risk factors (
30).
Second, all subtypes of maltreatment were associated with mental health problems, including experiences involving threat (e.g., physical, sexual, and emotional abuse) and deprivation (e.g., neglect), consistent with non-quasi-experimental evidence (
72). However, emotional abuse and institutional neglect were more strongly associated with mental health problems than some other types of maltreatment. The stronger association between emotional abuse and mental health problems was based on evidence from only three studies, but it supports findings from non-quasi-experimental studies (
1,
63,
73,
74). If causal, it might occur because parental criticism becomes internalized and directly leads to negative self-views and distress (
63,
73). However, noncausal mechanisms are also possible; for example, because emotional abuse is particularly likely to co-occur with other forms of maltreatment (
63,
74), stronger associations might reflect cumulative effects of other maltreatment types. The finding might also partly reflect recall bias, as two of the three included studies used retrospective reports of emotional abuse (
32,
42), which might be particularly influenced by current psychopathology given the more subjective nature of the experience (compared to other maltreatment types, e.g., sexual or physical abuse). Institutional neglect (experienced in Romanian orphanages) may also be strongly associated with mental health problems because 1) it reflects a particularly extreme, severe, and pervasive form of trauma and/or 2) it occurred during the first few years of life (e.g., between birth and 3 years), disrupting development of brain architecture and neurobiological systems that can affect psychopathology (
75).
Third, we did not find that effect sizes differed between studies assessing childhood maltreatment prospectively versus retrospectively. This may seem surprising, as prospective and retrospective measures identify largely different groups of individuals (
76) and retrospective reports have been found to be more strongly associated with psychopathology than prospective measures (
65,
77). Nevertheless, the overall pattern of our findings (i.e., visually larger effect sizes for retrospective vs. prospective measures) broadly supports these previous results. Such differences may be less pronounced in this meta-analysis because of heterogeneity between samples, as quasi-experimental data were not available to compare effect sizes between prospective and retrospective measures within the same samples, as in previous non-quasi-experimental research (
65,
77).
Our findings should be interpreted in the context of several limitations. First, each of the quasi-experimental approaches involves potential sources of bias (detailed in Table S3 in the
online supplement). For example, family-based designs do not account for nonfamilial confounding, panel data designs can be affected by time-varying confounding, propensity score approaches are liable to unmeasured confounding more generally, and natural experiments may not examine generalizable exposures (e.g., institutional neglect may be qualitatively different from maltreatment in the home). While we therefore cannot entirely rule out unmeasured confounding, convergent findings across multiple quasi-experimental methods provide stronger support for causal inference than findings from any individual method (i.e., triangulation [
58]). Second, the type of quasi-experimental method can be related to the type of maltreatment measure used. For example, twin and sibling difference studies primarily used retrospective self-reports to assess maltreatment (e.g.,
29,
32,
36,
54), likely because prospectively collected parent reports and official records detect minimal within-pair variation in maltreatment (
27,
78). In contrast, the natural experiment design of the ERA Study meant that official records were used to prospectively assess institutional neglect (as adoption out of the Romanian institutions was handled by the authorities). It was not possible to disentangle the type of quasi-experimental method from these measurement characteristics in a multivariate moderation analysis, because limited data were available for comparisons (e.g., only four of 84 effect sizes from twin/sibling studies did not use self-reports to assess maltreatment and mental health). Hence, we cannot rule out the possibility that some true moderation effects of measurement or design features were not detected because of suppression by other correlated variables. Third, we cannot draw firm conclusions about the specific effects of maltreatment types, because poly-victimization is common and studies rarely controlled for other co-occurring forms of maltreatment. Fourth, because of a lack of available data, we could not examine whether the findings were moderated by key factors, such as the timing of maltreatment, the time interval between maltreatment and psychopathology, and race or ethnicity. Further quasi-experimental research is needed to address these questions (e.g., utilizing cohorts with detailed temporal measures of maltreatment, and racially diverse samples). Finally, although our meta-analysis focused on quasi-experimental studies, other designs can also strengthen causal inference about the effects of maltreatment, such as experimental animal studies (
79) and high-quality prospective longitudinal studies (
80–
82).
Our findings have implications for research, clinical practice, and public health. Regarding future research, these findings highlight the importance of adopting rigorous quasi-experimental methods to test potentially causal effects of childhood maltreatment. Observational studies using conventional multiple regression approaches are likely to overestimate the causal effects of maltreatment, and caution should thus be applied when interpreting such findings. Of course, quasi-experimental methods may not completely alleviate bias, and researchers should also take steps to further strengthen causal inference, such as using longitudinal designs and prospective measures incorporating multiple informants. Future research capitalizing on these methods should provide valuable insights into the effects of childhood maltreatment on other outcomes (e.g., physical health, socioeconomic outcomes) and can be used to inform estimates of the economic costs of maltreatment (
83,
84).
Regarding clinical practice, to minimize risk of psychopathology in individuals exposed to maltreatment, interventions should adopt a holistic approach that addresses both the maltreatment experience (e.g., via trauma-focused cognitive-behavioral therapy [
85]) and wider risk factors for mental illness. Regarding public health, interventions that can prevent maltreatment (e.g., home visitation programs for high-risk families [
86,
87]) should also prevent a proportion of cases of mental health problems in the population. Thus, investing in these interventions is not only essential for children’s welfare, but could prevent long-term financial costs and suffering due to mental illness.