responding. Another reason may be that the sample is overrepresented by subjects associating with peers with high levels of delinquency, thus with a higher power to detect significant interactions in the upper bound. We cannot rule out the possibility that this same genotype (i.e., GG) may confer lower levels of externalizing in a more supportive peer environment compared to A-carriers, consistent with the differential susceptibility theory (Belsky et al. 2009). Indeed, when probing interactions in this sample with measures of positive peer involvement, a pattern consistent with differential susceptibility emerged (Trucco et al. 2014). In summary, we have demonstrated that the influence of peer delinquency upon externalizing behavior was moderated by genetic variation in GABRA2 that in turn may impact risk for substance use. Risk factors in alcohol initiation in adolescents include both externalizing behavior and affiliation with deviant peers (Donovan 2004). We also ruled out the possibility that genetic predisposition, in the form of the GABRA2 SNP, influenced the choice of peers by testing gene–environment correlation (rGE).
