Other underlying mechanisms (e.g., social mechanisms such as peer selection and influence) are likely to be involved in the relationship between early substance use onset and later substance abuse as well.13 For instance, the socially mediated hypothesis14 suggests that the genetic relationship between early onset and later alcohol-related problems is mediated by exposure to environmental risk (e.g., through peers). Research consistently has shown that there are social mechanisms which increase the risk for substance use.15 For example, peer use and peer support for substance use during adolescence have been found to positively predict problematic alcohol and marijuana use during early adulthood.16 Other studies have shown that even after controlling for gender, family income, and adolescents' baseline substance use, peer substance use predicts greater future use.17
