This study used a prospective design to examine G × E interaction between friends' alcohol use and DRD4 genotype in predicting alcohol use and heavy drinking from adolescence to later young adulthood. The analyses modeled the reciprocal processes of peer selection and socialization over time, as well as continuity in own and friends' alcohol use and any differences in these relationships by DRD4 polymorphism. The results revealed the presence of an interaction between DRD4 and friends' alcohol use in young adulthood, but not earlier in adolescence or emerging adulthood. Specifically, higher levels of alcohol use in early 30's were predicted by having more friends who drank heavily in late 20's among those with the high risk (long) DRD4 allele, but not among those with the low risk genotype. Similarly, alcohol use in early 30's was correlated with current friends' heavy use only among those with the high risk DRD4 allele, but not in those with two low risk alleles. Regardless of DRD4 genotype, the results revealed a developmental pattern of peer influence on both alcohol use and heavy drinking from
