These findings provide an explanation for the high level of comorbidity observed in national epidemiological surveys (SAMHSA, 2008) and the increased risk for dependence seen in relatives of family members with a family history of drug dependence, compared to relatives of family members without a drug dependence history. Given the data available in this study, we cannot speculate on the role of the role of cannabis as a gateway drug for alcohol or tobacco. However, based on earlier studies using this sample we know that (i) alcohol and tobacco are the primary substances used during adolescence (Young et al., 2002) and (ii) comorbid drug dependence is rare during adolescence and increases with age (Palmer et al., 2009). Furthermore, although comorbid drug dependence is uncommon during adolescence, common genetic and environmental effects explain some of that comorbidity (Young et al., 2006). Thus, reassessing the nature of the lifetime comorbidity in this sample after the respondents have had the time to encounter different substances has increased our ability to detect the latent dimension of risk. The common genetic and environmental influence on
