Twin studies demonstrate the importance of genetic factors to the liability of alcohol, tobacco, and cannabis dependence and have been used to explore comorbidity among substances (Agrawal and Lynskey, 2008), however, few studies have examined the nature of the liability to endorse Diagnostic and Statistical Manual of Mental Disorders (DSM) dependence symptoms across multiple licit and illicit substances. Bivariate twin studies have suggested largely overlapping genetic influences. True and colleagues (1999) reported a correlation of 0.68 between genetic factors for nicotine and alcohol dependence in a sample of 3300 male twins of the Vietnam Era Twin Registry. Similarly, Sartor and colleagues (2010) identified a genetic correlation of 0.61 using DSM-IV dependence symptoms of alcohol and cannabis in a sample of 6257 individuals from the Australian Twin Registry. Multivariate twin studies have also provided evidence for common and specific factors for 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
