Genetically-informative studies of cannabis use and related symptomatology have consistently produced evidence for substantial genetic influences on both use and dependence phenotypes (Agrawal and Lynskey, 2006). In a sample of female twins, Kendler and Prescott (1998) found that 79% of variance in heavy cannabis use and 62.3% in cannabis dependence could be explained by genetic factors. A twin study of illicit drug use in males by the same group produced nearly identical heritability estimates: 84% for heavy use and 58% for dependence (Kendler et al., 2000). Similar results have been reported in two other twin studies: Lynskey et al. (2002) found that 64.3% of variance in heavy use and 53-68% of variance in dependence was attributable to genetic factors and van den Bree et al. (1998) estimated heritability of dependence at 44.7%.
