Introduction to the Special Issue: The Interplay Between Genetic Factors and the Peer Environment in Explaining Children's Social Adjustment

Virtually all aspects of human development are subject to both genetic (i.e., heritable) and environmental influences that conjointly shape developmental outcomes through various mechanisms of gene-environment interplay. This special issue presents new research examining how the peer environment works together with genetic factors to influence children's and adolescents' social development. To this end, three studies utilize a quantitative genetic approach, whereas two others use molecular genetic methodologies. Covering a wide age range from early childhood to early adulthood, the studies examine different processes of gene-environment correlation and gene-environment interaction while focusing on dyadic and group-based peer experiences (e.g., friendships, peer acceptance, and peer victimization) as well as on positive and negative aspects of social development (e.g., prosocial leadership and aggression). Overall, the findings from the studies in this special issue clearly illustrate that we need to consider how genetics and the peer environment effects work together if we are to gain a more complete picture of children's and adolescents' social development.It is with great pleasure that I present this special issue: 'The Interplay Between Genetic Factors and the Peer Environment in Explaining Children's Social Adjustment. While the study of human development was long dominated by a mainly environment-focused behavioral perspective, there is now a wide consensus that virtually all aspects of human development are subject to both genetic (i.e., heritable) and environmental influences. However, the scientific study of heritability in individual differences in human behavior is not a recent phenomenon but began more than a hundred years ago. First evidence that individual differences in intelligence and behavior are heritable was provided by Charles Darwin's cousin, Sir Francis Galton (1822-1911), in his article Hereditary Talents and Character (Rushton, 1990). Galton was also the first to use a twin design to disentangle the effects of heritable and environmental factors on individual differences in behavior. The term genetics emerged only in 1909 with the rediscovery of Mendel's laws.Quantitative genetic studies based on twin, family, or adoption designs were the main tool of research into human behavioral genetics for several decades. In contrast to molecular genetic studies, quantitative genetic studies do not measure specific genes and often do not even include any specific measure of the environment. Instead, the relative strength of genetic and environmental effects is statistically inferred by examining the similarity of family members with varying degrees of genetic relatedness, such as identical and fraternal twins (Falconer, 1996). Findings from such studies have shown that basically all facets of human development, including social behavior such as aggression or withdrawal, are influenced by both genetic and environmental factors (Turkheimer, 2000). Now, advances in molecular genetics and the completion of the DNA sequencing of the human genome (Collins, Morgan, & Patrinos, 2004) make it possible to identify specific genes implicated in interindividual differences in social behavior. Many scholars initially expected that genetic and environmental influences on human development work independently of each other (Kidd, 1991). Others have argued, however, that genes and environments are likely to interact as well as to influence each other reciprocally (Allen, Knobloch, & Pasamanick, 1961; Plomin, DeFries, & Loehlin, 1977; Scarr & McCartney, 1983). In line with this notion, there is now a growing body of research showing that genetic and environmental effects work together to shape behavioral development through various mechanisms of gene-environment interplay.Most studies on the interplay of genetic factors with the social environment have focused on the family environment such as parenting behavior or child maltreatment (Bellani, Nobile, Bianchi, Van Os, & Brambilla, 2012; Burt, Klahr, Neale, & Klump, 2013; Thapar, Harold, Rice, Langley, & O'Donovan, 2007). …