Our results are consistent with twin studies that suggest that in adolescence the strong effect of the environment on externalizing is modulated by genetic effects (Kendler et al. 2011). Furthermore, these results are largely consistent with the literature supporting the role of peer influence as a predictor of adolescent externalizing behavior. These studies have shown that increased association with deviant peers at ages 12–14 years predict more serious forms of externalizing behavior (e.g., covert forms of antisocial behavior) (Granic and Patterson 2006) and socialization effects from peer drug use to adolescent substance use (Wills and Cleary 1999). Here, we prospectively examined the socialization effects in the interplay between peer delinquency and GABRA2 during adolescence on externalizing behavior, rule breaking, and aggression 3 years later (i.e., 15–17 years old). As expected, the association with deviant peers in early adolescence predicts externalizing and rule breaking behavior 3 years later. However, there was no main effect of GABRA2 SNPs, highlighting the importance of the social environment in order for the moderating effect of GABRA2 to emerge in this developmental period. Nevertheless, we could
