Abstract: 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, rangeage = 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 ρg = .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.
Keywords: child maltreatment, genetics, etiology, families, risk factors, self-report
This extended family study demonstrates that experiencing maltreatment during childhood is partly heritable. Heritability was not restricted to a specific type of maltreatment, and shared genetic factors contributed to abuse and neglect. Common and unique environmental factors explained a considerable proportion of phenotypic variance, and common environment had a greater effect on maltreatment in low-SES families.
Heritability estimates ranged from 30% for neglect to 62% for severe physical abuse. These findings suggest that child maltreatment is in part genetically mediated by child effects.
The finding that child factors contribute to maltreatment does not imply, however, that the responsibility for maltreatment perpetrated by parents lies with the child. It is the role of parents to respond appropriately to challenging child behavior, and they might need support to fulfill this role adequately in case of challenging child behaviors. Interventions may benefit from incorporating parent training that supports more effective strategies of responding to potentially challenging behavior. Results from earlier genetically informed studies (i.e., adoption and twin designs) provide support for an evocative role of externalizing problems in negative parenting and maltreatment (Marceau et al., 2013; O’Connor, Deater-Deckard, Fulker, Rutter, & Plomin, 1998; Schulz-Heik et al., 2010). The association between externalizing behavior and maltreatment may be bidirectional as maltreatment increases antisocial behavior over time, even when taking into account genetic effects (Jaffee, Caspi, Moffitt, & Taylor, 2004).
In the present study, we used a continuous variable of maltreatment ranging from “no maltreatment,” to “harsh parenting,” and to “maltreatment.” Most participants reported experiences of maltreatment at the lower end of the spectrum. Consequently, our findings may be restricted to more typical harsh parenting rather than maltreatment. However, we found that severe physical abuse seemed highly heritable, in contrast with an earlier twin study showing that harsh parenting, but not maltreatment, was heritable (Jaffee, Caspi, Moffitt, Polo-Tomas et al., 2004). Whereas Jaffee, Caspi, Moffitt, Polo-Tomas et al. (2004) assessed maltreatment up to the age of 5 years, the present study covered maltreatment to the end of adolescence, when individuals gain agency to shape their environment (Bergen, Gardner, & Kendler, 2007; Elkins, McGue, & Iacono, 1997), which may increase child-based genetic influences. Another explanation for the discrepancy might be that the present study used self-report, while in their twin study Jaffee, Caspi, Moffitt, Polo-Tomas et al. (2004) used mother report about the twins’ maltreatment histories, which may have led to an overestimation of shared environment because parents tend to perceive the environment of their children as more similar than the children themselves do (Wade & Kendler, 2000). Other studies using self-report measures also found experienced maltreatment to be partly heritable (Fisher et al., 2015; Schulz-Heik et al., 2009; South et al., 2015). Arguably, children may have a tendency to emphasize the difference between the way they themselves were treated and how their siblings were treated. However, since siblings completed the questionnaires independent of each other, it is unlikely that in our study, using self-report has strongly increased the similarity between siblings. Moreover, using a multi-informant approach that included parent reports when available, we see the same pattern of results with slightly higher estimates for heritability and common environment. This likely stems from a reduction of measurement error evident in the lower unique environment estimates.
Common and Unique Environment
The present findings suggest that similarity between siblings in terms of maltreatment experiences should not be attributed to genetic effects only but also to common environment. This points to the role of the family environment and is in line with studies showing that parental psychopathology, parenting stress, lack of social support, and larger family size are important risk factors for maltreatment. Low SES has repeatedly been shown to be associated with maltreatment (Euser et al., 2013; Sedlak et al., 2010; Slack, Holl, Mcdaniel, Yoo, & Bolger, 2004; Stith et al., 2009). In our study, SES was related to neglect. While some of these factors, such as family size, are difficult to change, addressing factors such as parenting stress and social support may have a particularly high payoff as they would benefit all children in the family.
For the etiology of maltreatment, it is important to not only understand what makes children growing up in the same family similar but also what makes them different in the experience of maltreatment—the unique environment (Plomin, 2011). We found an estimated influence of unique environment (including measurement error) of 29–42%, which concurs with previous studies (Fisher et al., 2015; Schulz-Heik et al., 2009). The importance of the unique environment points to the need for an individual child approach in addition to a family-centered approach when estimating risk. That is, it is important to improve the specific parent–child relationship. Nonetheless, specific unique environmental risk factors have remained elusive after taking measurement error into account (Deater-Deckard et al., 2001; Mullineauxa, Deater-Deckard, Petrillb, & Thompson, 2009). One suggested factor is that parents might perceive siblings as differently attractive or difficult, and thus trigger differential parenting (Burt, McGue, Iacono, & Krueger, 2006; Deater-Deckard, Smith, Ivy, & Petril, 2005; Feinberg & Hetherington, 2001; Reiss et al., 1995), which constitutes a potential target of intervention.
Genetic Correlation Between Abuse and Neglect
Our bivariate analysis indicates that the same common environmental factors are related to abuse and neglect. Approximately 50% of the genetic factors were overlapping, and the other 50% were uniquely related to abuse or neglect. This may suggest that some child factors put a child at risk of experiencing abuse but not of neglect and vice versa. Our findings illustrate why abuse and neglect often co-occur (Euser et al., 2013; Vachon, Krueger, Rogosch, & Cicchetti, 2015) notwithstanding etiological differences. Conversely, interventions may need to address abuse and neglect individually, even when they co-occur, since the heritable and unique environmental risk factors do not (fully) overlap.
Genotype × SES
Moreover, a genotype × SES interaction analysis demonstrated that in low-SES families, common environment explained more variance in experienced maltreatment than in high-SES families. Overall, low-SES families showed greater variance in experienced maltreatment, and our findings suggest that this can be attributed to common environment. Lower SES may add a range of common environmental factors negatively affecting child development. For instance, children from low-SES families experience more instability, more crowding at home, more pollution, and more danger in the neighborhood (Chen & Miller, 2013; Evans, 2004; Miller et al., 2009). Together, these factors may increase the risk of developing externalizing problems. On a population level, this suggests that fighting child poverty may have far-reaching preventive consequences.
Additive genetic variance, in absolute terms, remained stable across different levels of SES. Since overall variance decreased as SES increased, relative contribution of genetic variance component increased. Given the substantial effects of heritability this and previous studies have indicated, a more comprehensive exploration of environmental effects on heritability may uncover new intervention targets. A better understanding of the child traits mediating the heritable risk might offer insight into which environmental manipulations would be most effective in lowering heritable risk.
In order to interpret any variance component across a changing environment, it is important to consider changes in the other variance components. In a genotype-by-sex interaction study of physical activity behavior by Diego et al. (2015), the issue of the indeterminacy of environment-specific heritability was broached. The authors found that the heritability could be constant across an environmental contrast if the constituent variance components changed in the same direction and at the same rate. They also noted that it was theoretically possible that a nonconstant heritability across an environmental contrast could arise from a changing residual environment component in the face of a constant additive genetic variance. This concept is relevant to properly contextualizing our results with existing reports on the heritability of maltreatment. In particular, Schulz-Heik et al. (2009) and South, Schafer, and Ferraro (2015), respectively, reported a higher and lower proportion of the total phenotypic variance attributed to the shared environment relative to the heritability. Regarding our study, we can actually claim both scenarios because the shared environment variance component declined relative to a constant additive genetic variance from the low end of the SES spectrum to the high end.
Extended Family Design
For the current study, we decided to use an extended family design to add to the existing twin research. Extended family designs have more variability in genetic relatedness and common environment than twin designs. In addition, twins create a unique family constellation and parenting demands may be atypical when caring for two same-aged children (Olivennes, Golombok, Ramogida, & Rust, 2005). Consequently, results from twin studies may not be generalizable to typical family constellations.
Moreover, the extended family design decreases the confounding between genetic relatedness and shared environment compared to nuclear families (Almasy & Blangero, 2010; Diego, Kent, & Blangero, 2015). By including horizontal relationships (e.g., cousins, half-siblings), in addition to vertical relationships (e.g., grandparent–grandchild), a systematic correlation between genetic distance and age difference is eliminated. For instance, half-siblings and grandparent–grandchild pairs have the same genetic distance, but half-siblings tend to be similar in age whereas grandparent–grandchild pairs are not.
Limitations
A limitation of this study is the retrospective assessment of maltreatment; no conclusions about causality can therefore be drawn. For ethical reasons, research on maltreatment is generally incompatible with experimental designs except for intervention studies that combined with a prospective design can be highly informative. The present study assessed maltreatment retrospectively, and time between potential maltreatment and assessment varied. Moreover, estimates of unique environment should be interpreted with caution as it is impossible to disentangle unique environmental effects from measurement. It is interesting to note, however, that estimates of unique environment decreased when including parent reports for a multi-informant approach. This could point to a reduction in measurement error. Future research should strive to replicate these findings in a larger, representative sample and in other populations (e.g., non-Western). Estimates from quantitative genetic research are population-specific. Even if genetic variation is stable across populations (which we do not know), environmental variability will affect estimates for both heritability and environment because these estimates represent relative contributions (Plomin, 2018; Velden, 1997). Lastly, the present sample may have been too small to estimate moderator effects reliably (Glahn et al., 2010)—especially because the effect of SES on common environment was small and the moderation analysis was exploratory. Future studies should replicate the genotype × SES effect in larger samples as these findings suggest that environmental interventions can be particularly useful.
Implications
Ideally, interventions are based on empirically supported, theoretical frameworks of etiology. The current study suggests that such frameworks should incorporate the heritability of experiencing maltreatment and that interventions should address both heritable and environmental risk factors. More research is needed to determine how to best reduce those risk factors. Moreover, it would be useful to explore other environmental factors than SES and how they moderate heritability—preferably factors that can be the focus of interventions.
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