Child-driven genetic factors can contribute to negative parenting and may increase the risk of being maltreated. Experiencing childhood maltreatment may be partly heritable, but results of twin studies are mixed. In the current study, we used a cross-sectional extended family design to estimate genetic and environmental effects on experiencing child maltreatment. The sample consisted of 395 individuals (225 women; M age = 38.85 years, range age = 7–88 years) from 63 families with two or three participating generations. Participants were oversampled for experienced maltreatment. Self-reported experienced child maltreatment was measured using a questionnaire assessing physical and emotional abuse, and physical and emotional neglect. All maltreatment phenotypes were partly heritable with percentages for h 2 ranging from 30% ( SE = 13%) for neglect to 62% ( SE = 19%) for severe physical abuse. Common environmental effects ( c 2 ) explained a statistically significant proportion of variance for all phenotypes except for the experience of severe physical abuse ( c 2 = 9%, SE = 13%, p = .26). The genetic correlation between abuse and neglect was ρ g = .73 ( p = .02). Common environmental variance increased as socioeconomic status (SES) decreased ( p = .05), but additive genetic and unique environmental variances were constant across different levels of SES.

Each year, approximately 3.4–4.0% of children experience maltreatment in higher income countries (Euser et al., 2013; Sedlak et al., 2010). The etiology of maltreatment is complex, and a single causal pathway to maltreatment does not seem plausible. Rather, multiple risk and protective factors have been identified (Patwardhan, Hurley, Thompson, Mason, & Ringle, 2017). Heritable as well as environmental factors may contribute to maltreatment risk. Risk factors can be present at the level of the parent (e.g., psychopathology), the child (e.g., irritable temperament), or the family (e.g., socioeconomic adversities) and may interact with each other.

Risk factors related to the child have thus far been studied less (Stith et al., 2009) but may play an important role in the etiology of maltreatment. Parenting is a bidirectional process (Klahr et al., 2017), and child-driven factors can contribute to negative parenting (Avinun & Knafo, 2014; Davidov, Knafo-Noam, Serbin, & Moss, 2015). For instance, externalizing behavior (such as conduct problems, antisocial behavior, oppositional defiant disorder, and attention deficit hyperactivity disorder) may increase the risk of maltreatment and other types of victimization (Nobile et al., 2013; Stith et al., 2009).

As these phenotypes are partly heritable (Hicks, Krueger, Iacono, McGue, & Patrick, 2004; Nikolas & Burt, 2010; Porsch et al., 2016), they may genetically mediate the risk of experiencing maltreatment. One children-as-twin study tested this hypothesis and found that corporal punishment but not physical abuse was heritable in childhood, suggesting that genetic influences on the phenotype of experienced parenting may be limited to more normative parenting responses (Jaffee, Caspi, Moffitt, Polo-Tomas et al., 2004). However, when the same twin population was studied in adolescence, abuse and neglect were found to be heritable with additive genetic estimates of 71% and 47%, respectively (Fisher et al., 2015). In addition, evidence from two children-as-twin studies (one including adolescents and young adults and one including adults) suggests that approximately one quarter of the variance in experienced maltreatment can be attributed to genetic variation and to a lesser extent to common environment (Schulz-Heik et al., 2009; South, Schafer, & Ferraro, 2015). It should be noted, however, that in both studies, confidence intervals were large and more than half of the variance was explained by unique environmental factors including measurement error.

Importantly, these studies do not suggest that there are no parent effects on maltreatment. Parent effects are part of environmental effects which were consistently present in these studies. Moreover, these models are not deterministic—they do not suggest that children with a specific genetic layout will inevitably be maltreated. Rather, genetic factors (and potentially associated behavior) may increase the risk of being maltreated—especially when combined with environmental risk factors and a vulnerable parent.

Heritability may carry the negative connotation of a trait being fixed, but evidence is accumulating that the influence of heritability may be malleable by environmental factors, that is, gene-by-environment interactions. In psychological research, socioeconomic status (SES), in particular, has been associated with changes in the strength of heritability. The nature of these changes is not clear, however. Research on cognitive abilities has associated low SES with smaller genetic influences (Bates, Lewis, & Weiss, 2013) and greater shared environmental effects (Tucker-Drob, Rhemtulla, Harden, Turkheimer, & Fask, 2011). Similarly, the social push perspective suggests that genetic effects may be suppressed in high-risk environments (Middeldorp et al., 2014; Raine, 2002). In the context of gene-by-environment interaction, it becomes clear that the importance of understanding the heritability of maltreatment goes beyond providing an etiological framework. Namely, some interventions may be effective in reducing environmental risk of child maltreatment but not the genetic risk and vice versa. If research demonstrates that environmental factors such as SES affect the influence of heritability and environment differentially, interventions could be tailored to ensure that both are reduced.

The present study examined additive genetic and common and unique environmental effects on experienced abuse and neglect in an extended family design. This design has been applied previously to estimate the heritability of perpetrating maltreatment (Pittner et al., 2019). Extended family designs include family members beyond the nuclear family such as grandparents and cousins and across several generations. Extended family designs work under the assumption that, if a phenotype is heritable, individuals who are more closely related to each other will be more similar in a specific trait. Each family member shares a genetic relatedness with all other family members which generally lies between 6.25% and 50% (Almasy & Blangero, 2010). Lower levels of relatedness are possible if more distant relatives such as fourth cousins are included, and higher levels are possible if monozygotic twins are included.

The primary aim of this study was to compute heritability estimates for experienced maltreatment overall and for abuse and neglect separately. Our hypothesis was that, in line with most previous research (Fisher et al., 2015; Schulz-Heik et al., 2009; South et al., 2015), significant heritability components would be found independent of maltreatment type. Maltreatment was comprised of emotional and physical abuse and emotional and physical neglect—averaged across types. Overall, maltreatment, abuse, and neglect were treated as a continuous measures ranging from “no maltreatment” to “(severe) maltreatment.” We also examined whether severe physical abuse shows a heritability estimate similar to maltreatment. The aim was to test the idea postulated by Jaffee, Caspi, Moffitt, Polo-Tomas, and colleagues (2004) that child effects do not extend to more severe forms of maltreatment. We focused on physical abuse, rather than other types of maltreatment, in line with Jaffee, Caspi, Moffitt, Polo-Tomas et al.’s (2004) definition of the type of maltreatment that was central to their study. Second, we applied a multivariate approach to test the genetic and environmental overlap of abuse and neglect. Lastly, we explored whether SES moderates heritability of child maltreatment, similar to cognitive abilities displaying smaller genetic influences and larger shared environmental effects in children from low-SES backgrounds (Bates et al., 2013).

Method Sample The sample consisted of 395 individuals (225 women; M age = 38.85 years, range age = 7.50–88.42 years) from 63 families with two or three generations participating in the 3G Parenting Study. On average, 6.27 family members per family participated (range: 2–23; see Online Appendix S1, Online Appendix Table S1, and Figure 1). Participants were recruited from three Dutch participant pools: (1) the Netherlands Study of Depression and Anxiety (Penninx et al., 2008), (2) a study on parenting in low-SES families (Joosen, Mesman, Bakermans-Kranenburg, & Van IJzendoorn, 2013), and (3) the Longitudinal Internet Studies for the Social Sciences panel (Scherpenzeel & Toepoel, 2012). From two of these studies, we oversampled participants who reported having experienced maltreatment during childhood, and from the third study, all participants were invited. If the target participant agreed to take part in the study, family members of the target participant and of the target participant’s partner were invited to participate (parents [G1], children [G3], siblings [G2], nieces [G3], and nephews [G3]). Family members had to be at least 7.5 years of age to be invited. Families were included if at least two first-degree relatives from two generations agreed to participate (Pittner et al., 2019). Download Open in new tab Download in PowerPoint The following distribution for education was found for adult participants (≥18 years, n = 302): 6% elementary school, 19% lower vocational school, 40% advanced secondary education, and 28% college or university degree (6% unknown). The sample was economically diverse with the following percentages for yearly household incomes: 6.1% less than €15,000, 11.4% €15,000–€ 24,999, 12.9% €25,000–€ 34,999, 9.1% €35,000–€ 44,999, 8.6% €45,000–€ 54,999, 4.6% €55,000–€ 64,999, and 8.4% more than €65,000. The average household income in the Netherlands in 2014 was €59,600 (Statistics Netherlands, 2017). Procedure Nuclear families were invited to attend a 7-hr lab visit at the Leiden University Medical Center. Participants from the second generation came twice—once with their family of origin (parents and siblings) and once with their partner and children. A lab visit involved questionnaires, computer tasks, family interaction tasks, and the collection of saliva and hair samples. Informed consent was obtained from all participants. For participants under 18 years of age, parents cosigned informed consent. After each lab visit, child maltreatment questionnaires were checked for all children under 18 years of age (see Online Appendix S2). Ethical approval was obtained from the Ethics Committee of the Leiden University Medical Centre. Instruments Demographic information Age and gender were included as background variables. Participants of 18 years and older filled out a questionnaire with questions about household income and highest completed education. Yearly household income was measured on a 7-point scale ranging from (1) less than €15,000 to (7) more than €65,000. Due to changes in the Dutch educational system, first- and second-generation participants rated education on a 7-point scale and third-generation participants rated education on a 10-point scale. Both scales were rescaled to a 4-point scale. Based on standardized household income and standardized completed educational level, a composite household SES score was calculated. If data of two partners living in the same household were available, their scores were averaged for the household SES score. Children living with their parents shared their parents’ household SES score. Experienced child maltreatment Experienced child maltreatment was measured using a combination of the self-reported Parent–Child Conflict Tactics Scales (CTSPC; Straus, Hamby, Finkelhor, Moore, & Runyan, 1998) and the Childhood Trauma Questionnaire (CTQ; Bernstein et al., 1994; Thombs, Bernstein, Lobbestael, & Arntz, 2009). The following subscales were used: (1) Physical Assault (i.e., physical abuse, 13 items; CTSPC), (2) Psychological Aggression (i.e., emotional abuse, 5 items; CTSPC), (3) Physical Neglect (4 items; CTSPC), and (4) Emotional Neglect (6 items; CTSPC and CTQ). The Physical Abuse Scale consists of three subscales: Minor (5 items), Severe (4 items), and Very Severe (4 items) physical abuse. For consistency in response options, a 5-point scale ranging from 1 (never) to 5 (almost) always was used for all items. We assessed maternal and paternal behavior separately. For the separate subtypes, we first calculated averages for maltreatment perpetrated by mother and maltreatment perpetrated by father. Then, per subscale, the higher score of mother or father was included in the analyses. Internal consistencies of the subscales were as follows: α mother = .91, α father = .91 for physical abuse, α mother = .79, α father = .74 for emotional abuse, α mother = .62, α father = .54 for physical neglect, and α mother = .92, α father = .90 for emotional neglect. An overall “maltreatment” score was calculated by averaging subscale scores for all subtypes. Both mother and father scores correlated strongly with the overall maltreatment score, mother: r(393) = .89, p < .01 and father: r(389) = .85, p < .01, suggesting that the results were not driven by either mother or father. The “abuse” score was the average of physical and emotional abuse, r(393) = .67, p < .01, and the “neglect” score was the average of physical and emotional neglect, r(393) = .40, p < .01. The distribution for (very) severe physical abuse was highly skewed to the right (skewness = 4.59, SE = .12). Therefore, we distinguished between a group with severe or very severe physical abuse experiences (n = 155) and a group without such experiences (n = 240) for the additional analyses on abuse severity. The very severe physical abuse scale includes items such as “Grabbed around the neck and chocked” and “Burned or scolded on purpose.” For participants under 12 years of age, experienced maltreatment was assessed orally and questions about very severe physical abuse were omitted. For additional details, see Online Supplementary Material (Online Appendix S3). We did not assess sexual abuse in the current study because this maltreatment type has the lowest prevalence rate (Euser et al., 2013). In order to estimate heritability of sexual abuse, a larger sample size would have been necessary. Analyses Descriptive and correlational analyses were conducted using SPSS Statistics Version 23.0 (SPSS Inc.). All heritability analyses were performed in the statistical genetic analysis software SOLAR 8.1.1 (Almasy & Blangero, 1998). SOLAR is a comprehensive system for likelihood-based statistical analysis of variance components models. In twin research, high agreement has been found between SOLAR and other methods of estimating heritability (Kochunov et al., 2019). Heritability (h 2) is defined as the proportion of phenotypic variance attributable to additive genetic variance, estimated using a kinship matrix. In the kinship matrix, genetic relatedness for all possible participant pairs is listed. Heritability is estimated under a polygenic model (i.e., multiple gene inheritance), and significance is determined by comparing the log likelihoods of the models when the heritability parameter is estimated and when it is constrained to zero (see Online Appendix S4). In a first step, preliminary polygenic model analyses were performed for the phenotypes maltreatment, abuse, and neglect with age, age2, sex, age × sex, age2-by-sex, and SES as covariates. The residuals from these three models (one model per trait) were transformed using inverse normalization. In the following step, polygenic heritability analyses were performed for these new phenotypes. To estimate the common environmental variance (c 2), a household component was included in all models. Full- and half-siblings were coded as sharing or having shared the same household if they had grown up in the same household for at least 5 years (n = 146). Household was included in the analysis by adding a matrix in the prediction model with value 1 for all pairs of participants who shared a household and value 0 for all other pairs. This means that shared environment in the current study is defined as the environment that is shared by siblings growing up in the same household. It is likely that there is some dependence in environment between the parents’ environment and offspring’s environment—for instance, if parents grow up in poverty, their children are more likely to grow up in poverty as well. Part of this can be attributed to heritability—which would be estimated by the model. However, this similarity in poverty may partly also be attributed to environmental effects. In the current study, this is only modeled in as far as siblings are similarly affected by growing up in the same or similar environments but not in as far as parents and offspring may grow up in similar environments. This similarity in environments is difficult to model because parents and offspring do not grow up in the same household in the same way as siblings do, and any assumption on the similarity in environment would be speculative unless explicitly assessed. Moreover, this similarity may differ from family to family. In some cases, offspring may, for instance, maintain the social status of their parents, while in others, offspring will have higher or lower SES. Therefore, we assessed SES and controlled for it. Secondary analyses were conducted for physical and emotional abuse and emotional neglect following the same analysis steps to explore whether the results were specific one or more of these types of maltreatment. Physical neglect was excluded from the analyses as internal consistency was insufficient. Heritability of severe physical abuse was estimated using a liability threshold model for dichotomous phenotypes. The covariates age, age2, sex, age × sex, age2 × sex, SES and household were included in the analysis. In a sensitivity check, we repeated these analyses using multi-informant scores instead of self-report to measure experienced maltreatment. Multi-informant scores were computed as the averages of self-report and parent report when available (which was the case in 55% of the scores). A bivariate polygenic model analysis was conducted for abuse and neglect to estimate from the phenotypic covariance their constituent genetic and environmental correlations between the two traits. Further, we tested for potential genotype × SES interaction effects on overall maltreatment. Genotype × SES interaction arises if the additive genetic variance underlying the trait of interest changes with the environment (SES in the present case) or if the across-environment genetic correlation is less than 1 or if both conditions are true. The genotype × SES interaction model is a reparameterized version of the polygenic model in which the additive genetic and residual environmental variances are allowed to change as functions of SES by way of “change” parameters respectively denoted by γ g , γ c , and γ e , and in which the genetic correlation is expressed as an exponential decay function of pairwise differences in SES with parameter λ (Diego, Almasy, Dyer, Soler, & Blangero, 2003). We first compared the full genotype × SES interaction model to the polygenic model by a likelihood ratio test. If warranted by the results of this general comparison, we then compared the full genotype × SES interaction model to either of its constrained versions in which the additive genetic variance was constrained to be constant (γ g = 0) or in which the genetic correlation was constrained to be 1 (λ = 0). Next, common and unique environmental variances were constrained to be constant in turn (γ c = 0, γ e = 0).

Results The number of participants who experienced the various types of maltreatment never, once, or more than once is displayed in Online Appendix Table S2, and the distributions are shown in Figure S1. Descriptive statistics and correlations between all variables are reported in Online Appendix Table S3. Abuse and neglect were correlated, phenotypic covariance, r(393) = .56, p < .01. Men and women did not differ on experienced maltreatment, abuse, neglect, age, or SES (ps > .09). Older participants reported more maltreatment and lower SES (ps < .01). Participants with lower SES tended to report more neglect, r(393) = .13, p = .01, but not abuse, abuse: r(393) = −.04, p = .41, severe physical abuse: r(393) = −.06, p = .20. Heritability Analyses All maltreatment phenotypes were partly heritable (Table 1 and Online Appendix Table S4) with estimates ranging from .30 (SE = .13) for neglect to .62 (SE = .19) for severe physical abuse indicating that a statistically significant proportion of the phenotypic variance was explained by kinship. Overlapping 95% confidence intervals (CIs) indicate that heritability estimates were similar for all maltreatment phenotypes. Common environmental effects explained a statistically significant proportion of variance for all phenotypes except for severe physical abuse (c 2 = 0.09, SE = .13, p = .26). CIs overlapped for all maltreatment phenotypes. In a secondary analysis, we confirmed that these results were not specific to the physical or emotional dimension of maltreatment (Online Appendix Table S5). In addition to self-reports, parent reports were available for 218 participants. In a sensitivity check, we showed that CIs for self-report and multi-informant report overlapped (Online Appendix Table S6). Justifications and results of power analysis are reported in Online Appendix S3. Power to detect heritability and common environment was adequate to excellent (.77–.93) with the exception of common environmental effects on severe physical abuse for which power was only .17. Power was also calculated for the same traits under constrained heritability models where the heritability was constrained to .25, .48, and .71. Only a large heritability estimate leads to sufficient power for most types of maltreatment except for severe physical abuse that occurred rather infrequently in our sample. Replication is needed in larger (at risk) samples with higher prevalence of the various maltreatment types, in particular severe physical abuse. Table 1. Estimates of Genetic (h 2), and Common (c 2 ), Unique (e) Environmental Effects on Overall Maltreatment, Neglect, Abuse, and Severe Physical Abuse. View larger version Bivariate Heritability Analysis The bivariate analysis showed that the genetic correlation between abuse and neglect differed significantly from 0 (p = .02) as well as from 1 (p = .02) indicating partial pleiotropy (Figure 2). Pleiotropy occurs when the same genes contribute to different phenotypes. The genetic correlation was ρ g = .73, indicating 53% overlap. This means that shared as well as different genetic factors contribute to abuse and neglect. The common environmental correlation of ρ c 2 = .74 differed significantly from 0 (p < .01) but did not differ significantly from 1 (p = .08) indicating that there was substantial overlap in shared environmental factors accounting for variance in abuse and neglect. The unique environmental correlation was not significantly different from 0 (ρ e = .20, p = .40) showing that the unique environmental factors associated with abuse and with neglect are not the same. Download Open in new tab Download in PowerPoint Genotype × SES The genotype × SES model fitted the data significantly better than the main effects model (p < .001; Online Appendix Table S7 and Figure 3). Next, we compared the full genotype × SES model to its constrained alternatives (γ g = 0 or λ = 0, γ c = 0, and γ e = 0). Model fit decreased significantly after constraining γ c to zero (p = .048), indicating that the common environmental variance changed with SES. Specifically, common environmental variance increased as SES decreased (γ c = −1.11, SE = .40). Constraining γ g , λ, and γ e to zero did not lead to significant changes in model fit, indicating that additive genetic and unique environmental variances were constant across different levels of SES. Download Open in new tab Download in PowerPoint

Conclusion The present study shows that both genetic and environmental factors are critically involved in experiencing maltreatment. Findings further suggest that abuse and neglect share common pathways that are important because experiencing more than one type of maltreatment is related to worse outcomes (Vachon et al., 2015). Meta-analytic evidence suggests that in general, current interventions are insufficient in preventing maltreatment (Euser, Alink, Stoltenborgh, Bakermans-Kranenburg, & Van IJzendoorn, 2015). A better understanding of the transactional relationship between child and parent risk factors may be crucial in developing more targeted prevention measures. For instance, interventions can use video feedback to train parents in strategies to respond to specifically challenging problem behavior in children (Klein Velderman et al., 2006). The importance of intervention cannot be underestimated: Not only because the current study shows considerable associations with environmental factors but also because heritability does in no way preclude or limit the influence of environmental change.

Acknowledgments We are grateful to all the families that have invested their time by participating in this study and to the students whose contribution to the data collection was invaluable. We thank Rudi Westendorp for his contribution to the conception of the study.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The study was supported by the Netherlands Organization for Scientific Research (MB: VICI grant [no. 453-09-003]; LA: VIDI grant [no. 016.145.360]; MvIJ: NWO SPINOZA prize) and grants of Leiden University to initiate and support the Research Profile Area Health, Prevention and the Human Life Cycle awarded to MHvIJ, P. Assendelft, and B. van Hemert. ORCID iD

Katharina Pittner https://orcid.org/0000-0002-8954-3532 Lenneke R. A. Alink https://orcid.org/0000-0003-3459-0785 Supplemental Material

Supplemental material for this article is available online.