Despite the fact that low self-control and exposure to delinquent peers are two of the most robust and consistent predictors of crime, delinquency, and antisocial behavior, much remains unknown about what causes self-control to develop and what causes youths to befriend antisocial peers. This study estimated the relative effects of environmental and genetic factors on levels of self-control and contact with delinquent peers in a sample of twins from the National Longitudinal Study of Adolescent Health (Add Health). DeFries-Fulker analysis of the Add Health data revealed that both self-control and contact with drug-using friends were influenced by genetic factors and the nonshared environment, whereas the shared environment exhibited relatively small and inconsistent effects. Implications for self-control theory and social learning theory are discussed.