Two large-scale twin studies of illicit drugs showed that most genetic risk is common across the abuse of or dependence on different psychoactive substances6,7. One study examined the specificity of risk factors for both licit and illicit SUDs8 (Fig. 1). Genetic liabilities to licit and illicit SUDs are separable but highly correlated. Cannabis and cocaine dependence are strongly influenced by an illicit substance dependence factor. Loadings for alcohol and nicotine dependence on the licit substance dependence factor—that is, the degree to which the dependence symptoms for these drugs reflect the underlying common factor—are more modest, and the loading for caffeine dependence is low. Genetic influences specific to each substance are substantial for nicotine and caffeine, but less so for the other substances.
