Читать книгу The Wiley-Blackwell Handbook of Childhood Social Development - Группа авторов - Страница 53

In behavioral genetics, the environment is viewed as any influences on a specific phenotype (trait) that are not genetic (not related to DNA) and that contribute to individual differences in this phenotype in a population. Therefore, this definition of the environment is broader than the one that is usually used in psychology. For example, this definition may include influences in prenatal and postnatal periods, such as nutrition, illnesses, hormonal levels, and social relationships with family and peers.

One of the goals of behavioral genetic research is to identify specific environmental factors that contribute to individual differences in psychological traits, including social development (Reiss et al., 2000; Turkheimer & Waldron, 2000). In order to do this, genetically informative designs, such as twin and adoption designs, have been employed (Plomin & Daniels, 1987; Reiss et al., 2000; Rutter, 2006; Rutter et al., 2001; Turkheimer & Waldron, 2000). Many genetically informative studies use multiple informants (children, parents and other family members, peers, teachers, researchers) to measure environmental factors, such as parent–child relationships, sibling relationships, and relationships with peers and teachers.

The twin method allows for estimation of the relative contribution of two types of environment: shared (common) environment that contributes to the similarity between the two twins in a pair on a particular trait; and nonshared (unique) environment that contributes to the dissimilarity between the two twins. As twins grow up together they objectively share many aspects of their environment, including parents, socioeconomic conditions and often schools, classrooms, teachers, and peers (Plomin et al., 2013). The question of why children in the same family are very different from each other has been extensively researched in behavioral genetics (Plomin, 2011; Plomin et al., 2001; Plomin & Daniels, 1987). One of the findings is that objectively shared factors more often than not end up as nonshared effects. For example, socioeconomic status (SES) of the family, objectively shared by the two twins, may actually lead to the differences between the twins if they perceive or respond to this status differently (Plomin et al., 2001). A low SES may motivate one child to strive for achievement, but lead to lower motivation and achievement in another child. Such subjective reactions may form under the influence of many factors, including genetics (see the section “The Role of Gene–environment Interplay in Social Development”).

The results of a meta‐analysis of 43 studies demonstrated that nonshared environment accounts for ~50% of variability of many behavioral traits. However, none of the measured nonshared environmental factors (“objective” environments) has explained more than 2% of phenotypic variance (Turkheimer & Waldron, 2000). Since the publication of this meta‐analysis, multiple studies have been conducted searching for objective nonshared environmental factors involved in behavior of children and adolescents in different contexts (Asbury et al., 2003; Beaver, 2008; Deater‐Deckard et al., 2001). For example, a sample of 289 monozygotic twin pairs from the National Longitudinal Study of Adolescent Health (Add Health) in the United States was used to examine the effects of the nonshared familial environment on delinquent involvement, adult criminal behavior, levels of self‐control, and contact with antisocial peers (Beaver, 2008). Analysis revealed that one nonshared family environment – maternal disengagement – was associated with delinquent involvement and the development of self‐control. The remaining nonshared familial environments were not associated with social outcomes.

An extension of the twin design is the children‐of‐twins (COT) design that is based on samples of monozygotic (MZ) and dizygotic (DZ) twins with children. The offspring of MZ twins are as related to their parent’s co‐twin as they are to their own parent (they share 50% of their DNA). In contrast, the offspring of DZ twins share 25% of their genetic variance with their parent’s co‐twin. By comparing MZ avuncular correlations (correlations between uncle/aunt and niece/nephew) with DZ avuncular correlations, it is possible to estimate the role of genetic factors in explaining intergenerational associations. It is also possible to estimate the extent to which parent–child associations remain after accounting for genetic transmission. D'Onofrio et al. (2007), using this COT method, found that the effect of parental divorce on offspring substance use problems was significant even when genetic and other environmental risks were controlled for.

A large‐scale longitudinal study, the NEAD (Non‐shared Environment and Adolescent Development) in the United States includes data 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).