The goal of this dissertation is to investigate the roles of social environment, particularly, peers and family, and genetic factors in determining adolescent and young adult risky behaviors. The first essay addresses the endogenous selection of peers by using the (quasi) random assignment of college roommates. Empirical results suggest that peers are negatively associated with binge drinking, positively associated with marijuana use, and have no effects on risky sexual behaviors. The second essay evaluates the importance of genetic factor in mediating the selection of peers. The selection process starts from genes expressing certain personality traits such as risk-taking, novelty seeking and depression, in which these behaviors lead to substance use. Then adolescents select friends or are selected by friends who also engage in such behaviors (Cleveland et al., 2005). Estimates suggest that genetic factors do not seem to mediate the selection of peers in adolescent substance use. Finally, the last essay explores the role of genes further by examining the effects of gene-environment interactions on adolescent substance use. Results suggest that the effects of genes on adolescent substance use vary by social environments.
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