The primary aim of our study was to determine the degree to which the (lifetime) co-occurrence of heavy alcohol and cannabis use and the comorbidity of alcohol and cannabis dependence symptomatology are explained by common heritable and environmental factors. In addition, we sought to quantify the overlap in sources of variance between consumption and dependence symptomatology in these two substances of abuse. To address these aims, a series of genetic models were fitted, producing estimates of the magnitude of genetic and environmental influences on alcohol consumption and dependence symptoms and cannabis use and dependence symptoms as well as the associations between these influences across the four phenotypes.
