Hierarchical multiple regression analyses of questionnaire data reported by male twins in the Finnish Twin Cohort were used to evaluate genetic and social influences on use and abuse of alcohol. The similarity in drinking patterns of twin brothers is related to the frequency of their social interaction, and monozygotic co-twins are in more frequent social contact. But the stepwise multiple regressions show that the greater social interaction of identical twins cannot account for their greater similarity in drinking patterns. We conclude that genetic variance significantly contributes to population variance in the frequency, quantity, and density of social drinking.