In addition to the high within-substance correlations, we found a moderate degree of overlap in genetic influences across alcohol and cannabis in the respective use and dependence symptom phenotypes. Genetic factors were correlated at 0.675 for use and 0.613 for dependence symptomatology., but over half of inherited vulnerabilities to heavy use and to dependence symptoms were explained by substance-specific influences (57% for cannabis dependence symptoms; 53.9% for cannabis use) This etiological model is consistent with the high – but far from 100% – rate of co-occurrence for alcohol and cannabis-related problems (Degenhardt and Hall, 2003; Xian et al., 2008).The genetic correlation of 0.675 for use in our sample also coincides closely with the genetic correlation of 0.62 between problem alcohol use and problem cannabis use reported by Young et al. (2006), who examined these phenotypes in combination with nicotine dependence symptoms. Our estimates of overlap in heritable influences on dependence symptoms are highly consistent with Xian et al. (2008), who found that 42.4% of the variance in AD and 33.7% of the variance in cannabis dependence were accounted for by
