Analysing the contributions of genes and parent-child interaction to childhood behavioural and emotional problems: a model for the children of twins

Annual Research Review: Towards a deeper understanding of nature and nurture: combining family-based quasi-experimental methods with genomic data

Distinguishing between the effects of nature and nurture constitutes a major research goal for those interested in understanding human development. It is known, for example, that many parent traits predict mental health outcomes in children, but the causal processes underlying such associations are often unclear. Family-based quasi-experimental designs such as sibling comparison, adoption and extended family studies have been used for decades to distinguish the genetic transmission of risk from the environmental effects family members potentially have on one another. Recently, these designs have been combined with genomic data, and this combination is fuelling a range of exciting methodological advances. In this review we explore these advances - highlighting the ways in which they have been applied to date and considering what they are likely to teach us in the coming years about the aetiology and intergenerational transmission of psychopathology.

Do the Temperamental Characteristics of Both Mother and Child Influence the Well-Being of Adopted and Non-Adopted Children?

(1) Background: For decades, the temperaments of infants and small children have been a focus of studies in human development and been seen as a potential contributor to children’s developmental patterns. However, less is known about the interplay between the temperamental characteristics of mothers and their children in the context of explaining variations in developmental outcomes. The aim of our study was to explore the associations—with or without genetic links—of the temperaments and psychological distress of mothers and the temperaments of children with behavioral problems in a group of internationally adopted children and their adoptive mothers and in a group of non-adopted children and their mothers. (2) Methods: Data (n = 170) were derived from the ongoing Finnish Adoption (FinAdo) follow-up study. The children included were under the age of 7 years; 74 were adopted internationally through legal agencies between October 2010 and December 2016, and the remaining 96 were non-adopted children living with their birth parents (biological group) recruited from day-care centers. We used Mary Rothbart’s temperament questionnaires to assess temperament, the Child Behavior Checklist (CBCL) to obtain data on the children’s behavioral/emotional problems and competencies, and the General Health Questionnaire (GHQ) to assess parental psychological distress. The study was approved by the Ethics Committee of the Hospital District of Southwest Finland, and written informed consent was obtained from the parents and the children themselves. (3) Results: The negative affectivities of both mothers and children were associated with the total CBCL and with both internalizing and externalizing problem behaviors after adjusting for age, gender, and adoption status. Both relationships remained significant when tested simultaneously, suggesting additive effects. Maternal negative affect was associated with problem behavior irrespective of child extraversion/surgency. Child extraversion/surgency was associated with lower levels of all internalizing behavioral problems when adjusted for maternal sociability. Child negative affect was associated with all behavioral problem measures irrespective of maternal sociability or maternal psychological distress. Maternal distress was associated with child problem behaviors only in children with low extraversion/surgency. (4) Limitations: The sample size was relatively small, and the information was gathered solely with questionnaires. (5) Conclusions: The results of the study may be clinically significant. Child negative affect, maternal negative affect, and maternal experienced distress, combined with low child extraversion/surgency, may increase the risk of child problem behaviors in both adoptees and non-adoptees.

Twins and Causal Inference: Leveraging Nature's Experiment

In this review, we discuss how samples comprising monozygotic and dizygotic twin pairs can be used for the purpose of strengthening causal inference by controlling for shared influences on exposure and outcome. We begin by briefly introducing how twin data can be used to inform the biometric decomposition of population variance into genetic, shared environmental, and nonshared environmental influences. We then discuss how extensions to this model can be used to explore whether associations between exposure and outcome survive correction for shared etiology (common causes). We review several analytical approaches that can be applied to twin data for this purpose. These include multivariate structural equation models, cotwin control methods, direction of causation models (cross-sectional and longitudinal), and extended family designs used to assess intergenerational associations. We conclude by highlighting some of the limitations and considerations that researchers should be aware of when using twin data for the purposes of interrogating causal hypotheses.

Comparison of Adopted and Nonadopted Individuals Reveals Gene-Environment Interplay for Education in the UK Biobank

Polygenic scores now explain approximately 10% of the variation in educational attainment. However, they capture not only genetic propensity but also information about the family environment. This is because of passive gene-environment correlation, whereby the correlation between offspring and parent genotypes results in an association between offspring genotypes and the rearing environment. We measured passive gene-environment correlation using information on 6,311 adoptees in the UK Biobank. Adoptees' genotypes were less correlated with their rearing environments because they did not share genes with their adoptive parents. We found that polygenic scores were twice as predictive of years of education in nonadopted individuals compared with adoptees (R²s = .074 vs. .037, p = 8.23 × 10^-24). Individuals in the lowest decile of polygenic scores for education attained significantly more education if they were adopted, possibly because of educationally supportive adoptive environments. Overall, these results suggest that genetic influences on education are mediated via the home environment.

Revisiting the Children-of-Twins Design: Improving Existing Models for the Exploration of Intergenerational Associations

Datasets comprising twins and their children can be a useful tool for understanding the nature of intergenerational associations between parent and offspring phenotypes. In the present article we explore structural equation models previously used to analyse Children-of-Twins data, highlighting some limitations and considerations. We then present new variants of these models, showing that extending the models to include multiple offspring per parent addresses several of the limitations discussed. Accompanying the updated models, we provide power calculations and demonstrate with application to simulated data. We then apply to intergenerational analyses of height and weight, using a sub-study of the Norwegian Mother and Child Cohort (MoBa); the Intergenerational Transmission of Risk (IToR) project, wherein all kinships in the MoBa data have been identified (a children-of-twins-and-siblings study). Finally, we consider how to interpret the findings of these models and discuss future directions.

The Maternal Emotional Climate Predicts Twin Sibling Relationship Quality

We investigated the association between maternal expressed emotion and twin relationship quality, after controlling for a maternal questionnaire measure of the mother-child relationship. This was explored within a community sample of 156 mothers and their two young twin children (M child age = 3.69 years; SD child age = 0.37). Mothers reported on the twin-twin relationship and the mother-child relationship via questionnaire. They were also interviewed about each child using the innovative Preschool Five Minute Speech Sample (Daley et al., 2003), which yields information about relative positive:negative maternal expressed emotion. Mothers who expressed more family-wide positive emotion and less family-wide negative emotion also reported more positivity, but not negativity, within the twin relationship - even when controlling for questionnaire reports of the mother-child relationship. Counter to expectations, discrepancies in mothers' expressed emotion between their twins also predicted more positive sibling relationships. Our findings corroborate the well established spill-over effect, whereby families are viewed as emotional units of interdependent individuals, none of whom can be understood in isolation from one another. Most importantly, the Preschool Five Minute Speech Sample provides information about mothering that questionnaire reports may not, and thus it is a useful tool in better understanding the twin family system.

Effects of parenting quality on adolescents' personality resemblance to their parents. The TRAILS study

This study considers the development of resemblance between 741 adolescents and their biological parents, across six NEO-PI-R personality traits known to be important in psychological problems: anger-hostility, impulsiveness, vulnerability, assertiveness, excitement-seeking, and self-discipline. We modelled the association between perceived parental warmth and rejection at age eleven and personality resemblance to parents at about age sixteen. Parenting experienced during early adolescence was related to the degree and direction in which adolescents resembled their parents five years later in life. Rejection, especially from fathers, significantly predicted a smaller resemblance to both the parents. Girls were more strongly affected by parental quality than boys, and there was some indication that adolescents responded in opposite ways to parenting from mothers and fathers. This study is a first step in uncovering the complex interplay between parenting, gender, and the current generation's ability to develop personality traits independent from the previous generation.

Translational Epidemiologic Approaches to Understanding the Consequences of Early-Life Exposures

Prominent developmental theories posit a causal link between early-life exposures and later functioning. Yet, observed associations with early exposures may not reflect causal effects because of genetic and environmental confounding. The current manuscript describes how a systematic series of epidemiologic analyses that combine several genetically-informative designs and statistical approaches can help distinguish between competing theories. In particular, the manuscript details how combining the use of measured covariates with sibling-comparisons, cousin-comparisons, and additional designs can help elucidate the sources of covariation between early-life exposures and later outcomes, including the roles of (a) factors that are not shared in families, including a potential causal effect of the exposure; (b) carryover effects from the exposure of one child to the next; and (c) familial confounding. We also describe key assumptions and how they can be critically evaluated. Furthermore, we outline how subsequent analyses, including effect decomposition with respect to measured, plausible mediators, and quantitative genetic models can help further specify the underlying processes that account for the associations between early-life exposures and offspring outcomes.

Proceeding From Observed Correlation to Causal Inference: The Use of Natural Experiments

This article notes five reasons why a correlation between a risk (or protective) factor and some specified outcome might not reflect environmental causation. In keeping with numerous other writers, it is noted that a causal effect is usually composed of a constellation of components acting in concert. The study of causation, therefore, will necessarily be informative on only one or more subsets of such components. There is no such thing as a single basic necessary and sufficient cause. Attention is drawn to the need (albeit unobservable) to consider the counterfactual (i.e., what would have happened if the individual had not had the supposed risk experience). Fifteen possible types of natural experiments that may be used to test causal inferences with respect to naturally occurring prior causes (rather than planned interventions) are described. These comprise five types of genetically sensitive designs intended to control for possible genetic mediation (as well as dealing with other issues), six uses of twin or adoptee strategies to deal with other issues such as selection bias or the contrasts between different environmental risks, two designs to deal with selection bias, regression discontinuity designs to take into account unmeasured confounders, and the study of contextual effects. It is concluded that, taken in conjunction, natural experiments can be very helpful in both strengthening and weakening causal inferences.

The relationship between parental depressive symptoms and offspring psychopathology: evidence from a children-of-twins study and an adoption study

BACKGROUND

Parental depressive symptoms are associated with emotional and behavioural problems in offspring. However, genetically informative studies are needed to distinguish potential causal effects from genetic confounds, and longitudinal studies are required to distinguish parent-to-child effects from child-to-parent effects.

METHOD

We conducted cross-sectional analyses on a sample of Swedish twins and their adolescent offspring (n = 876 twin families), and longitudinal analyses on a US sample of children adopted at birth, their adoptive parents, and their birth mothers (n = 361 adoptive families). Depressive symptoms were measured in parents, and externalizing and internalizing problems measured in offspring. Structural equation models were fitted to the data.

RESULTS

Results of model fitting suggest that associations between parental depressive symptoms and offspring internalizing and externalizing problems remain after accounting for genes shared between parent and child. Genetic transmission was not evident in the twin study but was evident in the adoption study. In the longitudinal adoption study child-to-parent effects were evident.

CONCLUSIONS

We interpret the results as demonstrating that associations between parental depressive symptoms and offspring emotional and behavioural problems are not solely attributable to shared genes, and that bidirectional effects may be present in intergenerational associations.

Behavioral Genetic Approaches and Family Theory

Family theories have been founded on research that cannot discriminate genetic and environmental influences and, consequently, most theories do not have highly developed models of gene-environment interplay in families. Behavioral genetic approaches, which can identify gene-environment interplay, have typically not been driven by family theories and have lacked adequate measurement of family processes. In this article, the authors describe behavioral genetic mechanisms and methods using representative examples of research on family processes, highlighting the advantages of an integrated approach. New directions in research and theory driven by an integrated approach are discussed.

Gene-environment correlation underlying the association between parental negativity and adolescent externalizing problems

Studies of adolescent or parent-based twins suggest that gene-environment correlation (rGE) is an important mechanism underlying parent-adolescent relationships. However, information on how parents' and children's genes and environments influence correlated parent and child behaviors is needed to distinguish types of rGE. The present study used the novel Extended Children of Twins model to distinguish types of rGE underlying associations between negative parenting and adolescent (age 11-22 years) externalizing problems with a Swedish sample of 909 twin parents and their adolescent offspring and a U.S.-based sample of 405 adolescent siblings and their parents. Results suggest that evocative rGE, not passive rGE or direct environmental effects of parenting on adolescent externalizing, explains associations between maternal and paternal negativity and adolescent externalizing problems.

Critical need for family-based, quasi-experimental designs in integrating genetic and social science research

Researchers have identified environmental risks that predict subsequent psychological and medical problems. Based on these correlational findings, researchers have developed and tested complex developmental models and have examined biological moderating factors (e.g., gene-environment interactions). In this context, we stress the critical need for researchers to use family-based, quasi-experimental designs when trying to integrate genetic and social science research involving environmental variables because these designs rigorously examine causal inferences by testing competing hypotheses. We argue that sibling comparison, offspring of twins or siblings, in vitro fertilization designs, and other genetically informed approaches play a unique role in bridging gaps between basic biological and social science research. We use studies on maternal smoking during pregnancy to exemplify these principles.

Maternal age at first birth and offspring criminality: using the children of twins design to test causal hypotheses

Teenage childbirth is a risk factor for poor offspring outcomes, particularly offspring antisocial behavior. It is not clear, however, if maternal age at first birth (MAFB) is causally associated with offspring antisocial behavior or if this association is due to selection factors that influence both the likelihood that a young woman gives birth early and that her offspring engage in antisocial behavior. The current study addresses the limitations of previous research by using longitudinal data from Swedish national registries and children of siblings and children of twins comparisons to identify the extent to which the association between MAFB and offspring criminal convictions is consistent with a causal influence and confounded by genetic or environmental factors that make cousins similar. We found offspring born to mothers who began childbearing earlier were more likely to be convicted of a crime than offspring born to mothers who delayed childbearing. The results from comparisons of differentially exposed cousins, especially born to discordant monozygotic twin sisters, provide support for a causal association between MAFB and offspring criminal convictions. The analyses also found little evidence for genetic confounding due to passive gene-environment correlation. Future studies are needed to replicate these findings and to identify environmental risk factors that mediate this causal association.

Resilience as a dynamic concept

The concept of resilience has as its starting point the recognition that there is huge heterogeneity in people's responses to all manner of environmental adversities. Resilience is an inference based on evidence that some individuals have a better outcome than others who have experienced a comparable level of adversity; moreover, the negative experience may have either a sensitizing effect or a strengthening "steeling" effect in relation to the response to later stress or adversity. After noting the crucial importance of first testing for the environmental mediation of risk through "natural experiments," findings are reviewed on "steeling effects" in animal models and humans. Gene-environment interaction findings are considered, and it is noted that there is some evidence that the genetic influences concerns responsivity to all environments and not just bad ones. Life course effects are reviewed in relation to evidence on turning point effects associated with experiences that increase opportunities and enhance coping. Attention is drawn to both research implications and substantive findings as features that foster resilience.

Current parental depression and offspring perceived self-competence: a quasi-experimental examination

A genetically-informed, quasi-experimental design was used to examine the genetic and environmental processes underlying associations between current parental depressive symptoms and offspring perceived self-competence. Participants, drawn from a population-based Swedish sample, were 852 twin pairs and their male (52 %) and female offspring aged 15.7 ± 2.4 years. Parental depressive symptoms were measured using the Center for Epidemiological Studies Depression scale. Offspring perceived self-competence was measured using a modified Harter Perceived Competence Scale. Cousin comparisons and Children of Twins designs suggested that associations between maternal depressive symptoms and offspring perceived self-competence were due to shared genetic/environmental liability. The mechanism responsible for father-offspring associations, however, was independent of genetic factors and of extended family environmental factors, supporting a causal inference. Thus, mothers and fathers may impact offspring perceived self-competence via different mechanisms and unmeasured genetic and environmental selection factors must be considered when studying the intergenerational transmission of cognitive vulnerabilities for depression.

Prenatal substance use, prenatal stress and offspring behavioural outcomes: considerations for future studies

BACKGROUND

Numerous studies have been conducted in which the most common forms of maternal substance use during pregnancy (smoking, drinking or using cannabis) and maternal stress during pregnancy and her offspring's developmental or behavioural outcome have been the focus of interest. These studies seem to suggest that any perturbation caused by maternal substance use or maternal stress during fetal development may have (enduring) effects on offspring behaviour. However, recent developments in research methodology used to examine these associations question whether these prenatal influences actually cause altered offspring outcomes.

AIMS

This review provides a short overview of previous studies in this field of research, some methodological issues particularly involved in studies that focus on the association between maternal substance use or stress during pregnancy and offspring's outcomes. Furthermore, it introduces several new approaches that have been applied recently to test these associations.

RESULTS

Studies that applied these designs to disentangle prenatal influences from associated or inherited factors consistently show an effect of prenatal substance use exposure on birth weight, but yield little evidence for causal effects on behaviour. In contrast, prenatal stress may have a causal effect on some aspects of behaviour, although only one study thus far has been able to differentiate heritable factors from environmental factors.

Paternal Alcoholism and Offspring Conduct Disorder: Evidence for the ‘Common Genes’ Hypothesis

Not only are alcoholism and externalizing disorders frequently comorbid, they often co-occur in families across generations; for example, paternal alcoholism predicts offspring conduct disorder just as it does offspring alcoholism. To clarify this relationship, the current study examined the ‘common genes’ hypothesis utilizing a children-of-twins research design. Participants were male monozygotic (MZ) and dizygotic (DZ) twins from the Vietnam Era Twin Registry who were concordant or discordant for alcohol dependence together with their offspring and the mothers of those offspring. All participants were conducted through a structured psychiatric interview. Offspring risk of conduct disorder was examined as a function of alcoholism genetic risk (due to paternal and co-twin alcohol dependence) and alcoholism environmental risk (due to being reared by a father with an alcohol dependence diagnosis). After controlling for potentially confounding variables, the offspring of alcohol- dependent fathers were significantly more likely to exhibit conduct disorder diagnoses than were off- spring of nonalcohol-dependent fathers, thus indicating diagnostic crossover in generational family transmission. Comparing offspring at high genetic and high environmental risk with offspring at high genetic and low environmental risk indicated that genetic factors were most likely responsible for the alcoholism–conduct disorder association. The observed diagnostic crossover (from paternal alcoholism to offspring conduct disorder) across generations in the context of both high and low environmental risk (while genetic risk remained high) supported the common genes hypothesis.

Revisiting the Children of Twins: Can They Be Used to Resolve the Environmental Effects of Dyadic Parental Treatment on Child Behavior?

The children of twins (COT) design has been proposed as an alternative to the adoption study to resolve the direct effects of parental treatment from secondary parent–child association due to genetic factors. The basic analytical approach compares the parent–offspring correlation with the correlation between children and the monozygotic (MZ) twins of their parents. We show that a significant difference between these correlations does not imply direct environmental causality when the measured parental treatment in question is dyadic, that is, influenced by both parents even when mating is random. Nongenetic causal effects yield very similar patterns of correlation to secondary genetic effects on dyadic treatment variables. The fact that many candidate environments, such as parental divorce, are dyadic gives reason to question the interpretation of their correlations with behavior in the children of twins.

A genetically informed study of associations between family functioning and child psychosocial adjustment

Research has documented associations between family functioning and offspring psychosocial adjustment, but questions remain regarding whether these associations are partly due to confounding genetic factors and other environmental factors. The current study used a genetically informed approach, the Children of Twins design, to explore the associations between family functioning (family conflict, marital quality, and agreement about parenting) and offspring psychopathology. Participants were 867 twin pairs (388 monozygotic; 479 dizygotic) from the Twin and Offspring Study in Sweden, their spouses, and children (51.7% female; M = 15.75 years). The results suggested associations between exposure to family conflict (assessed by the mother, father, and child) and child adjustment were independent of genetic factors and other environmental factors. However, when family conflict was assessed using only children's reports, the results indicated that genetic factors also influenced these associations. In addition, the analyses indicated that exposure to low marital quality and agreement about parenting was associated with children's internalizing and externalizing problems and that genetic factors also contributed to the associations of marital quality and agreement about parenting with offspring externalizing problems.

Parental criticism and externalizing behavior problems in adolescents: the role of environment and genotype-environment correlation

Genetic factors are important for the association between parental negativity and child problem behavior, but it is not clear whether this is due to passive or evocative genotype-environment correlation (rGE). In this study, we applied the extended children-of-twins model to directly examine the presence of passive and evocative rGE as well as direct environmental effects in the association between parental criticism and adolescent externalizing problem behavior. The cross-sectional data come from the Twin and Offspring Study in Sweden (N = 909 pairs of adult twins) and from the Twin Study of Child and Adolescent Development (N = 915 pairs of twin children). The results revealed that maternal criticism was primarily due to evocative rGE emanating from their adolescent's externalizing behavior. On the other hand, fathers' critical remarks tended to affect adolescent problem behavior in a direct environmental way. This suggests that previously reported differences in caretaking between mothers and fathers also are reflected in differences in why parenting is associated with externalizing behavior in offspring.

Gene - Environment Interplay, Family Relationships, and Child Adjustment

This paper reviews behavioral genetic research from the past decade that has moved beyond simply studying the independent influences of genes and environments. The studies considered in this review have instead focused on understanding gene - environment interplay, including genotype - environment correlation ( rGE) and genotype × environment interaction (G × E). Studies have suggested that rGE is an important pathway through which family relationships are associated with child adjustment. Also important are direct causal influences of family relationships on child adjustment, independent of genetic confounds. Other studies have indicated that genetic and environmental influences on child adjustment are moderated by different levels of family relationships in G × E interactions. Genetically informed studies that have examined family relations have been critical to advancing our understanding of gene - environment interplay.

A quasi-experimental study of maternal smoking during pregnancy and offspring academic achievement

The current study, based on all births in Sweden from 1983 to 1991 (N = 654,707), explored the processes underlying the association between smoking during pregnancy (SDP) and offspring school grades and mathematic proficiency at age 15. The analyses compared relatives who varied in their exposure to SDP and who varied in their genetic relatedness. Although SDP was statistically associated with academic achievement (AA) when comparing unrelated individuals, the results suggest that SDP does not cause poorer academic performance, as full siblings differentially exposed to SDP did not differ in their academic scores. The pattern of results suggests that genetic factors shared by parents and their offspring help explain why offspring exposed to SDP have lower levels of AA.

Internalizing behavior in adolescent girls affects parental emotional overinvolvement: a cross-lagged twin study

The aim of this study was to examine the direction and the etiology of the association between different parenting styles (parental emotional overinvolvement [EOI] and parental criticism) and internalizing behavior from adolescence to early adulthood. A longitudinal genetically informative cross-lagged design was applied to a population-based sample of Swedish twins contacted at age 16-17 (n = 2369) and at age 19-20 (n = 1705). Sex-limitation modelling revealed different effects for boys and girls. For girls, genetic influences on internalizing problems at age 16-17 independently explained 2.7% of the heritability in parental EOI at age 19-20. These results suggest that emotionally overinvolved and self-sacrificing parental behavior stems in part from daughters (but not sons) genetic predisposition for internalizing behavior. These findings highlight the importance of genetically influenced child-driven effects underlying the parenting-internalizing association, and clarify that the role of such effects may differ depending on sex, type of parenting and developmental period.

Biosocial Influences on the Family: A Decade Review

The past decade brought a remarkable increase in the number and quality of biosocial studies of family processes. The current review summarizes recent advances in biosocial family research by providing key exemplars of emerging research paradigms. Research in the past decade has substantiated the claim in the previous Decade Review (Booth, Carver, & Granger, 2000) that bidirectional influences between all levels of analysis are paramount. There is an emerging consensus that integrating factors at multiple biological and social levels is highly informative. Because ignoring biological factors often will underestimate mediating or moderating mechanisms, the review provides recommendations for biosocial family research. We also highlight the need for researchers who understand complex family environments to lend their expertise to biosocial studies.

Genetic and environmental influences on the transmission of parental depression to children's depression and conduct disturbance: an extended Children of Twins study

BACKGROUND

Despite the increased risk of depression and conduct problems in children of depressed parents, the mechanism by which parental depression affects their children's behavioral and emotional functioning is not well understood. The present study was undertaken to determine whether parental depression represents a genuine environmental risk factor in children's psychopathology, or whether children's depression/conduct can be explained as a secondary consequence of the genetic liability transmitted from parents to their offspring.

METHODS

Children of Twins (COT) data collected on 2,674 adult female and male twins, their spouses, and 2,940 of their children were used to address whether genetic and/or family environmental factors best account for the association between depression in parents and depression and conduct problems in their children. Data collected on juvenile twins from the Virginia Twin Study of Adolescent Behavioral Development (VTSABD) were also included to estimate child-specific genetic and environmental influences apart from those effects arising from the transmission of the parental depression itself. The fit of alternative Children of Twin models were evaluated using the statistical program Mx.

RESULTS

The most compelling model for the association between parental and juvenile depression was a model of direct environmental risk. Both family environmental and genetic factors accounted for the association between parental depression and child conduct disturbance.

CONCLUSIONS

These findings illustrate how a genetically mediated behavior such as parental depression can have both an environmental and genetic impact on children's behavior. We find developmentally specific genetic factors underlying risk to juvenile and adult depression. A shared genetic liability influences both parental depression and juvenile conduct disturbance, implicating child conduct disturbance (CD) as an early indicator of genetic risk for depression in adulthood. In summary, our analyses demonstrate differences in the impact of parental depression on different forms of child psychopathology, and at various stages of development.

The mediating effect of parental neglect on adolescent and young adult anti-sociality: a longitudinal study of twins and their parents

The causes of correlation between parental treatment and offspring behavior are ambiguous since genetic and social factors are correlated in typical family studies. The problem is complicated by the need to characterize the effects of genes and environment on both juvenile and adult behavioral outcomes. A model is developed for the resemblance between juvenile and adult twins and their parents that allows some of these effects to be resolved. Data on childhood adversity, parental anti-social behavior, and longitudinal adult and juvenile anti-social behavior were obtained from 1,412 families of adolescent and young adult twins. A structural model is fitted that allows for the effects of genetic and social transmission of information from parents to children. Environmental effects of parents may be mediated through measured features of the home environment. Parameters were estimated by diagonal weighted least squares applied to the 33 distinct polychoric correlations between relatives and between variables within and between ages. Sub-hypotheses were tested. Results confirmed that effects of genes and environment were both highly significant. Genetic effects were large in juveniles and largely age and sex-specific. Approximately 30% of the variation due to the shared environment was due to the effect of childhood adversity. The remaining shared environmental effects are unexplained. Adversity is affected significantly by maternal anti-social behavior. The correlation between paternal ASP and adversity may be explained by antisocial fathers selecting (or creating) antisocial mothers. All significant environmental effects of parental ASP are mediated through the measure of adversity. Though transmission of ASP is both genetic and social, passive genotype-environment correlation is very small. Assortative mating for ASP has barely detectable consequence for the genetic correlation between siblings. The longitudinal study of twins and their parents makes it possible to demonstrate there is a direct causal effect of childhood adversity on child conduct disorder over and above any indirect genetic correlation.

Testing the equal environments assumption in the Children of Twins design

In a Children of Twins (COT) design, the environmental and genetic risk of a child is, in part, dependent upon the status of the father and the father's cotwin. The logic of the COT method breaks down if the zygosity of the twin pair is confounded with the environment provided to the child (a version of the Equal Environment Assumption, EEA). If MZ twin fathers see each other more often than DZ twin fathers, and a child's uncle is the affected twin in discordant pairs, this could increase the environmental risk of children of MZ over that of DZ discordant twins. The current study was designed to test the EEA in the COT design, specifically in children of alcohol and drug dependent fathers. Results indicated that MZ twins did have more contact than DZ twins. Regression analyses were conducted to predict child externalizing symptom counts from father's zygosity group status, level of contact with father's cotwin, and their interaction. Results found no significant interaction between father's zygosity and the higher level of cotwin contact (seen in MZ twins) in predicting several measures of offspring externalizing risk. The results of this study suggested that the COT design does not confound zygosity with differences in environmental risk exposure, findings that support the validity of the EEA within this research context.

Estimating fetal and maternal genetic contributions to premature birth from multiparous pregnancy histories of twins using MCMC and maximum-likelihood approaches

The analysis of genetic and environmental contributions to preterm birth is not straightforward in family studies, as etiology could involve both maternal and fetal genes. Markov Chain Monte Carlo (MCMC) methods are presented as a flexible approach for defining user-specified covariance structures to handle multiple random effects and hierarchical dependencies inherent in children of twin (COT) studies of pregnancy outcomes. The proposed method is easily modified to allow for the study of gestational age as a continuous trait and as a binary outcome reflecting the presence or absence of preterm birth. Estimation of fetal and maternal genetic factors and the effect of the environment are demonstrated using MCMC methods implemented in WinBUGS and maximum likelihood methods in a Virginia COT sample comprising 7,061 births. In summary, although the contribution of maternal and fetal genetic factors was supported using both outcomes, additional births and/or extended relationships are required to precisely estimate both genetic effects simultaneously. We anticipate the flexibility of MCMC methods to handle increasingly complex models to be of particular relevance for the study of birth outcomes.

Moderate use of alcohol, tobacco and cannabis during pregnancy: new approaches and update on research findings

Interest in fetal origins of adverse offspring outcomes has grown extensively in the last decade. This has resulted in many published studies focusing on exposure in utero to substances and human offspring outcomes. Exposure to maternal substance use in pregnancy is believed to be a preventable hazard, and is therefore a main issue for public health concern and policy. However, an important question in human studies remains whether prenatal substance use exposure has an aetiological role in pathways to adverse developmental and behavioural outcomes via teratological effects. Recent insights and developments in research methodology will aid the adequate and more refined testing of associations between prenatal substance use and offspring outcomes. In particular, novel approaches could assist in disentangling the exposure to substance effects from correlated risk factors. The purpose of this manuscript is therefore to provide an overview of methodological issues involved in studies that focus on the association between maternal substance use during pregnancy and offspring's outcomes, to describe novel approaches to test these associations, and present some examples of new and well-designed studies and discuss their findings.

Testing different types of genotype-environment correlation: an extended children-of-twins model

This study presents an extended children-of-twins model, which allowed the authors to test the direction of the association between parenting and child adjustment. Three mechanisms were examined: direct phenotypic influence of parenting on child behavior (controlling for both parental and child genotype), passive genotype-environment correlation, and evocative genotype-environment correlation. This model was tested with Monte Carlo simulations. The authors generated data sets consisting of 1,000 twin parent pairs together with their children and 1,000 twin children pairs together with their parents. These simulated data sets were then used to estimate the model, and the procedure was repeated 1,000 times. The simulation results showed that this model recovered the true values of parameters with high precision. The model was also applied to an observed data set to analyze, as a first example, the association between maternal emotional overinvolvement and child internalizing problems. The results showed that this association was best explained by evocative genotype-environment correlation.

The cross-generational mother-daughter-aunt-niece design: establishing validity of the MDAN design with NLSY fertility variables

Using National Longitudinal Survey of Youth (NLSY) fertility variables, we introduce and illustrate a new genetically-informative design. First, we develop a kinship linking algorithm, using the NLSY79 and the NLSY-Children data to link mothers to daughters and aunts to nieces. Then we construct mother-daughter correlations to compare to aunt-niece correlations, an MDAN design, within the context of the quantitative genetic model. The results of our empirical illustration, which uses DF Analysis and generalized estimation equations (GEE) to estimate biometrical parameters from NLSY79 sister-sister pairs and their children in the NLSY-Children dataset, provide both face validity and concurrent validity in support of the efficacy of the design. We describe extensions of the MDAN design. Compared to the typical within-generational design used in most behavior genetic research, the cross-generational feature of this design has certain advantages and interesting features. In particular, we note that the equal environment assumption of the traditional biometrical model shifts in the context of a cross-generational design. These shifts raise questions and provide motivation for future research using the MDAN and other cross-generational designs.

A Children of Twins Study of parental divorce and offspring psychopathology

BACKGROUND

Although parental divorce is associated with increased substance use and internalizing problems, experiencing the separation of one's parents may not cause these outcomes. The relations may be due to genetic or environmental selection factors, characteristics that lead to both marital separation and offspring functioning.

METHOD

We used the Children of Twins (CoT) Design to explore whether unmeasured genetic or environmental factors related to the twin parent, and measured characteristics of both parents, account for the association between parental divorce and offspring substance use and internalizing problems.

RESULTS

The association between parental divorce and offspring substance use problems remained robust when controlling for genetic and environmental risk from the twin parent associated with parental divorce, and measured characteristics of both parents. The results do not prove, but are consistent with, a causal connection. In contrast, the analyses suggest that shared genetic liability in parents and their offspring accounts for the increased risk of internalizing problems in adult offspring from divorced families.

CONCLUSIONS

The study illustrates that unmeasured genetic and environmental selection factors must be considered when studying parental divorce. In explaining associations between parental divorce and young-adult adjustment, our evidence suggests that selection versus causal mechanisms may operate differently for substance abuse (a causal relation) and internalizing problems (an artifact of selection). The CoT design only controls for the genetic and environmental characteristics of one parent; thus, additional genetically informed analyses are needed.

Intergenerational transmission of childhood conduct problems: a Children of Twins Study

CONTEXT

The familial nature of childhood conduct problems has been well documented, but few genetically informed studies have explicitly explored the processes through which parental conduct problems influence an offspring's behavior problems.

OBJECTIVE

To delineate the genetic and environmental processes underlying the intergenerational transmission of childhood conduct problems.

DESIGN

We used hierarchical linear models to analyze data from a Children of Twins Study, a quasiexperimental design, to explore the extent to which genetic factors common to both generations, unmeasured environmental factors that are shared by twins, or measured characteristics of both parents confound the intergenerational association.

SETTING

Participants were recruited from the community and completed a semistructured diagnostic telephone interview.

PARTICIPANTS

The research used a high-risk sample of twins, their spouses, and their young adult offspring (n = 2554) from 889 twin families in the Australian Twin Registry, but the analyses used sample weights to produce parameter estimates for the community-based volunteer sample of twins. Main Outcome Measure Number of conduct disorder symptoms.

RESULTS

The magnitude of the intergenerational transmission was significant for all offspring, though it was stronger for males (effect size [Cohen d] = 0.21; 95% confidence interval, 0.15-0.17) than females (d = 0.09; 95% confidence interval, 0.05-0.14). The use of the Children of Twins design and measured covariates indicated that the intergenerational transmission of conduct problems for male offspring was largely mediated by environmental variables specifically related to parental conduct disorder (d = 0.13; 95% confidence interval, 0.02-0.23). In contrast, the intergenerational transmission of conduct problems was not because of environmentally mediated causal processes for female offspring (d = - 0.09; 95% confidence interval, - 0.20 to 0.03); a common genetic liability accounted for the intergenerational relations.

CONCLUSIONS

The mechanisms underlying the intergenerational transmission of conduct problems depend on the sex of the offspring. The results are consistent with an environmentally mediated causal role of parental conduct problems on behavior problems in males. Common genetic risk, however, confounds the entire intergenerational transmission in female offspring.

Relationships between parental negativity and childhood antisocial behavior over time: a bidirectional effects model in a longitudinal genetically informative design

This study examined the direction and etiology underlying the relationships between parental negativity and early childhood antisocial behavior using a bidirectional effects model in a longitudinal genetically informative design. We analyzed parent reports of parental negativity and early childhood antisocial behavior in 6,230 pairs of twins at 4 and 7 years of age. Results from a cross-lagged twin model contribute to the understanding of the mechanisms underlying the bidirectional processes involved in parental negativity and childhood antisocial behavior. Specifically, the findings of this study suggest that the association between parenting and child antisocial behavior is best explained by both parent-driven and child-driven effects. We found support for the notion that parent's negative feelings towards their children environmentally mediate the risk for child antisocial behavior. We also found evidence of genetically mediated child effects; in which genetically influenced antisocial behavior evoke parental negativity towards the child.

Aggression as a mediator of genetic contributions to the association between negative parent-child relationships and adolescent antisocial behavior

Previous research suggests that the association between conflictual parent-child relationships and maladjustment among adolescents is influenced by genetic effects emanating from the adolescents. In this study, we examined whether these effects are mediated by childhood aggression. The data come from the Twin study of CHild and Adolescent Development (TCHAD), a Swedish longitudinal study including 1,314 twin pairs followed from age 13-14 to 16-17. Early adolescent aggression, parental criticism, and delinquency in later adolescence were rated by parents and children at different time points. Multivariate genetic structural equation models were used to estimate genetic and environmental influences on these constructs and on their covariation. The results showed that approximately half of the genetic contribution to the association between parental criticism and delinquency was explained by early adolescent aggression. It suggests that aggression in children evokes negative parenting, which in turn influences adolescent antisocial behavior. The mechanism proposed by these findings is consistent with evocative gene-environment correlation.

Marital conflict and conduct problems in Children of Twins

The Children-of-Twins design was used to test whether associations between marital conflict frequency and conduct problems can be replicated within the children of discordant twin pairs. A sample of 2,051 children (age 14-39 years) of 1,045 twins was used to estimate the genetic and environmental influences on marital conflict and determine whether genetic or environmental selection processes underlie the observed association between marital conflict and conduct problems. Results indicate that genetic and nonshared environmental factors influence the risk of marital conflict. Furthermore, genetic influences mediated the association between marital conflict frequency and conduct problems. These results highlight the need for quasiexperimental designs in investigations of intergenerational associations.

Gene-environment interdependence

Behavioural genetics was initially concerned with partitioning population variance into that due to genetics and that due to environmental influences. The implication was that the two were separate and it was assumed that gene-environment interactions were usually of so little importance that they could safely be ignored. Theoretical considerations suggested that that was unlikely to be true and empirical findings are now accumulating on the demonstrated and replicated biological interactions between identified common single genetic variants and the operation of environmentally mediated risks. The paper outlines the evidence and considers why it is changing concepts in ways that matter.

Challenges in genetic studies of the etiology of substance use and substance use disorders: introduction to the special issue

Misuse of psychoactive substances is associated with substantial costs to users and to society. A growing literature suggests individual differences in vulnerability to develop substance related problems are influenced to a large degree by genetic factors. We review the evidence from genetic epidemiologic and molecular genetic studies of problematic use of alcohol, tobacco, and other drugs, then discuss the challenges for the next generation of studies of genetic influences on substance use. These challenges are addressed in the remaining papers of this special issue. The papers cover a variety of approaches, substances, and non-human as well as human studies, but are united by their focus on going beyond heritability estimates to address the mechanisms and processes underlying the development of substance use and substance related problems, including measurement, precursors of substance abuse, stages of substance involvement, and specificity of genetic influences.

Measured Gene-Environment Interactions in Psychopathology: Concepts, Research Strategies, and Implications for Research, Intervention, and Public Understanding of Genetics

There is much curiosity about interactions between genes and environmental risk factors for psychopathology, but this interest is accompanied by uncertainty. This article aims to address this uncertainty. First, we explain what is and is not meant by gene-environment interaction. Second, we discuss reasons why such interactions were thought to be rare in psychopathology, and argue instead that they ought to be common. Third, we summarize emerging evidence about gene-environment interactions in mental disorders. Fourth, we argue that research on gene-environment interactions should be hypothesis driven, and we put forward strategies to guide future studies. Fifth, we describe potential benefits of studying measured gene-environment interactions for basic neuroscience, gene hunting, intervention, and public understanding of genetics. We suggest that information about nurture might be harnessed to make new discoveries about the nature of psychopathology.

Gene-environment interplay and psychopathology: multiple varieties but real effects

Gene-environment interplay is a general term that covers several divergent concepts with different meanings and different implications. In this review, we evaluate research evidence on four varieties of gene-environment interplay. First, we consider epigenetic mechanisms by which environmental influences alter the effects of genes. Second, we focus on variations in heritability according to environmental circumstances. Third, we discuss what is known about gene-environment correlations. Finally, we assess concepts and findings on the interaction between specific identified genes and specific measured environmental risks. In order to provide an understanding of what may be involved in gene-environment interplay, we begin our presentation with a brief historical review of prevailing views about the role of genetic and environmental factors in the causation of mental disorders, and we provide a simplified account of some of the key features of how genes 'work'.

A genetically informed study of marital instability and its association with offspring psychopathology

Parental divorce is associated with a number of emotional and behavioral problems in young-adult offspring, but theoretical and empirical considerations suggest that the relation may be partially or fully accounted for by passive gene-environment correlation or environmental selection characteristics. The current study used the Children of Twins Design to explore whether shared environmental or genetic factors confound the relationship between parental marital instability and measures of psychopathology. Comparisons of the offspring of adult twins in Australia on 3 factors of abnormal behavior, including drug and alcohol, behavioral, and internalizing problems, suggest that environmental influences associated with divorce account for the higher rates of psychopathology. The results are consistent with a causal connection between marital instability and psychopathology in young-adult offspring.

The new look of behavioral genetics in developmental psychopathology: gene-environment interplay in antisocial behaviors

This article reviews behavioral-genetic research to show how it can help address questions of causation in developmental psychopathology. The article focuses on studies of antisocial behavior, because these have been leading the way in investigating environmental as well as genetic influences on psychopathology. First, the article illustrates how behavioral-genetic methods are being newly applied to detect the best candidates for genuine environmental causes among the many risk factors for antisocial behavior. Second, the article examines findings of interaction between genes and environments (G x E) associated with antisocial behavior, outlining steps for testing hypotheses of measured G x E. Third, the article envisages future work on gene-environment interplay, arguing that it is an interesting and profitable way forward for psychopathology research.

Environmentally mediated risks for psychopathology: research strategies and findings

OBJECTIVE

To consider the research design requirements needed to provide a rigorous test of environmental mediation hypotheses and to summarize the main findings from research using such designs.

METHOD

Selective review of empirical evidence dealing with psychopathology.

RESULTS

There is robust evidence of environmentally mediated risks for psychopathology. There are major individual differences in people's responses to risk experiences. Effects are often dependent on genetic susceptibility (operating through gene-environment interactions).

CONCLUSIONS

Many of the risks deriving from adverse experiences are reliant on nature-nurture interplay, and one of the main research needs concerns the diverse effects of the environment on the organism.