We found that genetic and shared environmental influences on alcohol use and perceived peer alcohol use are strongly correlated from early adolescence into early adulthood, and these influences account for the majority of covariation (56–91%) between the two phenotypes. The influence of unique environmental factors – those factors that twins do not share – impact the relationship between PEER and SELF via reciprocal causal pathways. Specifically, non-shared environmental factors influencing one’s own drinking also indirectly impacted exposure to peers’ drinking behaviors via social selection. This effect was reciprocal, in that non-shared environmental factors influencing peer affiliation – and thus exposure to peers’ drinking behaviors – indirectly influenced one’s own concurrent (and subsequent) drinking through social influence. Thus, while we observed modest evidence of theoretically causal social selection and social influence, genetic and environmental correlation dominated the etiology underlying the association between alcohol consumption and peer alcohol problems.
