Chunk #18 — Basic methodology of genetic epidemiology: an overview of twin studies — Principle 4: Our genetic predispositions shape our behavior in part by influencing our environments
Separating etiological risk factors into genetic and environmental factors is in some ways a misnomer, as we know that many environmental factors show evidence of genetic influence (Kendler and Baker, 2007). A systematic review of heritability of measures of the environment, to include stressful life events, parenting and family environment, social support, peer interactions, and marital quality, found that most measures of the environment yield heritability estimates in the range of 15–35%. The heritability of environmental factors reflects the fact that individuals select and shape their environmental experiences based in part on their own genetically influenced proclivities. This becomes particularly relevant in adolescence, as individuals have more freedom to select and shape their environments. Research on peer affiliation/deviance is illustrative of these unfolding processes. Twin studies and other genetically informative designs yield evidence of genetic influence on peer deviance (Kendler et al., 2008a; McGue et al., 2006) and these genetic influences on peer group deviance increase across adolescence, accounting for only 30% of the variance in peer group deviance at 8–11 years of age, but steadily rising to account for
