The primary research question of this study was whether the liability to dependence across alcohol, tobacco, and cannabis could be explained by either a single common genetic risk factor or a genetic factor acting upon a latent mechanism that brings about vulnerability to developing dependence symptoms on an array of substances. Our results are similar to Xian et al.‘ s (2008) earlier report that suggested a single underlying mechanism for vulnerability to dependence on nicotine, alcohol, and cannabis dependence in adult males, as well as other multivariate twin studies of comorbid illicit drug dependence or problematic use (Tsuang et al., 1998; Kendler et al., 2003). This study confirms the existence of a latent dimension of liability to drug dependence symptoms in males and females, and the presence of substance specific genetic and environmental factors that account for variation in each substance that is unexplained by the latent dimension of risk. Notably, this study utilized structured missingness to exclude dependence information from non-substance users and experimenters, who might have been scored as “non-dependent/asymptomatic” using a diagnostic phenotype, thereby improving our ability
