Our investigation of genetic and environmental contributions to use and dependence symptomatology for alcohol and cannabis revealed significant associations both across and within substances. On the phenotypic level, it produced further support for the strong link between heaviness of use and dependence symptoms for both substances as well as the high degree of comorbidity between AD and cannabis dependence. Through biometrical modeling we estimated heritability for each of the four phenotypes as well as both within- and cross-substance overlap in genetic and environmental influences on use and dependence symptoms, revealing the substantial role of common heritable factors in observed associations.
