Phenotypic correlations among alcohol consumption, cannabis use, AD symptoms, and cannabis dependence symptoms are reported in Table 5; additive genetic and unique environmental correlations among the four phenotypes are shown in Table 6. As is evident from Table 6, our results provide strong support for within-substance and cross-substance overlap in heritable influences. For alcohol consumption and dependence symptoms, rG = 0.953; 90% of the genetic variance in alcohol consumption was shared with AD symptoms. A similarly high overlap in heritable factors was observed for cannabis use and dependence symptoms: rG = 0.979. Cross-substance genetic correlations were more modest, but still highly significant. Genetic correlations of 0.675 and 0.613 were found for alcohol consumption and cannabis use and for AD and cannabis dependence symptoms, respectively. As expected, given the high within-substance genetic correlation between use and dependence symptoms, genetic correlations were very similar for AD symptoms and cannabis use, rG = 0.621 and for alcohol use and cannabis dependence symptoms, rG = 0.655 as they were for cross-substance correlations of the same measure.
