from more than 700 families (adolescents and their parents) and allows: assessment of the differences in experiences of children from the same families; investigation of the links between the differences in experience and behavioral outcomes; and the determination of the causal directions of these links (Plomin et al., 2001; Reiss et al., 2000). A number of NEAD studies have focused on sociobehavioral traits, including, adjustment and relationships among family members (Neiderhiser et al., 2007). Different aspects of children’s environments have been studied, such as relationships with teachers, peers, and friends. The results have shown significant nonshared environmental influences on many behavioral traits in adolescents. However, specific measures of nonshared environmental factors (e.g., parent–child relationships, asymmetrical sibling relationships, peer relationships) were not significant (Spotts et al., 2001).
The effects of nonshared environment on externalizing problems and positive social engagement were demonstrated in middle childhood (mean age 6 years) using a monozygotic (MZ) twin differences design, in a sample 77 pairs of MZ twins (Mullineaux et al., 2009). The results indicated that differential maternal warmth and negativity – self‐perceived and observed by others – are important aspects of sibling differentiation for both problematic and adaptive behaviors during middle childhood.
To summarize, quantitative behavioral genetic research has improved our understanding of the ways environmental factors influence individual differences in psychological traits. However, there is a problem with identifying specific environmental factors contributing to the variance in complex traits, the so‐called “missing environment” problem. This problem is similar to the “missing heritability” problem – the problem with identifying specific genetic variants contributing to phenotypic variance (as discussed in the section “The Role of Genetic Factors in Social Development”). It seems that each specific environmental factor has only a small effect on a trait, and that environmental influences act in a cumulative manner. Nevertheless, the fact that specific environmental factors have not been identified does not negate their significance, but simply indicates that causal mechanisms of environmental influences might be more complex than was previously assumed (Rutter, 2006; Rutter et al., 2001).
The Role of Gene–environment Interplay in Social Development
Genetic and environmental factors do not work in isolation from each other, but rather co‐act during the development. This gene–environment co‐action or interplay includes gene–environment correlation (rGE) and gene–environment interaction (GxE).
Gene–environment correlation
Gene‐environment correlation (rGE) refers to the phenomenon whereby a person’s exposure to the environment depends on their genetic characteristics. As a result, some genotypes are observed more often in one environment than in another, because a person chooses, selects, and modifies their environment according to their genetic propensities. For example, a person with genetic propensities for antisocial behavior is more likely to experience the environment that can facilitate antisocial behavior, for example socializing with antisocial peers.
A systematic review of 55 studies of 35 environmental influences, including stressful life events, family environment, social support, and peer relationships, found that these experiences were partly influenced by genetic factors, with heritability estimates between 7% and 39% (with average of 27% across all environmental measures) (Kendler & Baker, 2007). How do genes influence environments? Environmental factors are subject to genetic influences since genetic variants can influence a person’s behaviors that are related to change, modification, and creation of specific environments. There are three types of rGE: passive, reactive (or evocative), and active. The main difference between these three types results from the source of environmental influences.
In the case of passive rGE, the source of environmental influences are parents. For example, parents with antisocial behavior, which is partly heritable, can pass on some genetic risk factors for antisocial behavior to their children. At the same time, parental antisocial behavior can also be related to family dysfunction. Therefore, children of parents with antisocial behavior may experience dual disadvantage – from both genetic and environmental risk factors for antisocial behavior, resulting from a passive rGE (Rutter, 2006).
Adoption studies can be used to test for passive rGE effects in the association between overall family functioning and child antisocial behavior. The adoption method is based on the fact that adopted children and their adoptive parents are not genetically related and, on average, only share a common postnatal environment, therefore the adoptive family environment is free from passive rGE. A stronger association in biological families in comparison to adoptive families is suggestive of passive rGE (Leve et al., 2010). For example, one adoption study found that the association between family functioning and adolescent externalizing problems was greater in biological than adoptive families, confirming the important role of passive rGE (McGue et al., 1996). However, studies examining specifically the role of parenting practices (rather than overall family functioning) indicate that the effects of passive rGE on the association between parenting and child and adolescent antisocial behavior are small to nonexistent (Harold et al., 2013; Marceau et al., 2012; Narusyte et al., 2011). In contrast, their results highlighted the role of evocative (reactive) rGE, the phenomenon explained in the section “Possible Implications of Behavioral Genetic Research.”
The classical adoption method (“adoption at birth”) cannot be used to disentangle prenatal environmental influences (e.g., maternal smoking during pregnancy, maternal depression during pregnancy) from genetic influences, because biological mothers provide both genes and prenatal environment to their children (i.e., passive rGE with prenatal environment). A novel adoption design (“adoption at conception”) allows a separation of the influences of prenatal environment from genetic influences on the child’s development. The design is possible because of modern in vitro fertilization (IVF) technologies that are used to help people with fertility problems to have a baby. During IVF, an egg is removed from the woman’s ovaries and fertilized with sperm in a laboratory. The fertilized egg is then returned to the woman’s womb to grow and develop. During this procedure, an egg and sperm can be used from biological parents, or alternatively, from egg and/or sperm donors. The “adoption at conception” design is based on the comparison of pairs of parents and children with different degrees of genetic relatedness: both parents are genetically related to their child (homologous IVF); only father is genetically related to the child (egg donation); only mother is genetically related to the child (sperm donation); neither of the parents is genetically related to the child (embryo donation) (Rice et al., 2009; Thapar et al., 2007). If a prenatal factor is associated with the child’s trait in both genetically related and genetically unrelated parent–child pairs, then this factor is more likely to be environmental (not influenced by common genetic factors). If a prenatal factor is associated with the child’s trait only in genetically related parent–child pairs, but not in genetically unrelated parent–child pairs, then it is likely to involve passive rGE.
An example of an “adoption at conception” study is the Cardiff IVF study. The study participants were families who had conceived a child through one of the assisted reproductive methods and were subsequently recruited through a number of different fertility clinics that agreed to participate (18 clinics in the United Kingdom and 1 in the United States). In total, 779 families with children born following successful IVF treatment were included. Using data from this study, researchers found that the associations maternal smoking during pregnancy and antisocial behavior of the child (at 4–10 years) were likely to be influenced by genetic factors shared by mothers and children, proving the important role of passive rGE in this association (Rice et al., 2009).
In the case of evocative rGE, an individual’s genetically influenced characteristics evoke particular environmental experiences or responses. For example, children exhibiting externalizing behavior would elicit negative parenting consistent with their genetic propensities towards
