Peers have been referred to as an important environmental risk factor for adolescent substance use and problem use. However, our results indicate that the situation is more complicated. We found that genetic influences explained about 30% of the variation of twin reports of friends' alcohol use. This is compatible with a peer selection process whereby twins seek out particular peers or peers seek out the twin with whom to be friends, which is partly genetically influenced. The results could also be explained by the adolescents' individual characteristics, given that peer selection and friendship maintenance are influenced by attitudes, personality and behaviour [12,13]. Both attitudes and personality have been shown to be, at least in part, genetically influenced [44,45].
