substance use disorders. In an examination of several comorbidity models, Rhee and colleagues (2006) compared alternative models of comorbid drug problems and identified a model that suggested that the comorbidity between alcohol and illicit drug problems was best interpreted as alternative manifestations of a common underlying trait. Young and colleagues (2006) used a large subset of community adolescent twins to identify a common genetic liability among alcohol, tobacco, and cannabis problem use (i.e., 1+ symptoms of abuse/dependence). Kendler and colleagues (2007) examined the structure of genetic and environmental risk factors for symptoms of dependence on five licit and illicit psychoactive substances in 4866 adult male and female twins and identified a complex architecture made up of highly intercorrelated genetic factors that differentially loaded unto licit and illicit substances. Using a design very similar to the present study, Xian et al. (2008) computed the contribution of common and specific genetic and environmental factors to nicotine, alcohol, and cannabis dependence in male veterans and concluded that the comorbidity between the three substances was attributable to common and specific genetic factors, as well as unique environmental effects. Overall, despite the knowledge we have gained thus far, additional studies are still necessary to determine
