A central sociological problem is the extent to which genetics and the environment influence human behavior. Studies of twins are a core method in attempts to disentangle and to determine the comparative strength of genetic and environmental influences on psychosocial outcomes. A critical assumption of twin studies is that both monozygotic "identical" twins and dizygotic "fraternal" twins share common social environments. Therefore, any greater similarity of monozygotic than dizygotic twins is attributed to genetic influences. This paper tests the equal environment assumption by examining the extent to which greater concordance of adolescent monozygotic compared to dizygotic twins results from social, as well as genetic, influences. Bivariate comparisons indicate that monozygotic twins show greater similarity than dizygotic twins in socially-based characteristics including physical attractiveness, time spent in each other's company, the overlap in friendship networks, and friends' use of alcohol. Multivariate analyses indicate that measures of the social environment sometimes reduce or eliminate apparent genetic effects. In comparison with genetic indicators, social variables are usually stronger predictors of depression and alcohol use and abuse. These findings suggest that past twin studies could overstate the strength of genetic influences because some similarities in behavior among monozygotic compared to dizygotic twins stem from social influences.