More recently, twin studies have examined the heritability of lifetime stimulant use, dependence, and abuse using liability threshold model fitting (Kendler et al. 1999, 2003, 2000). In their most recent study, Kendler et al. (2005) estimated heritability for lifetime use of stimulant drugs excluding cocaine to be 0.42, and heritability for lifetime use of cocaine to be 0.70. They also found substantial familial environmental contributions to the variance (i.e., 0.20) for lifetime use of other stimulant drugs, but not for cocaine use. Specific environmental effects, which are distinguished from shared familial environmental effects, were estimated at 0.38 for other stimulant use and 0.30 for cocaine use. In addition to this study, a study of male twin war veterans yielded similar results (Tsuang et al. 1996). That study estimated the heritability of stimulant abuse (including cocaine abuse) based on DSM-III-R criteria (American Psychiatric Association 1987) to be 0.44, but with 0.49 of the variance contributed by specific environmental effects, and no contribution of familial environment in the best-fit model. Although this was a slightly higher estimate of heritability than by Kendler et al. (2005), both studies suggest a fairly large contribution of genetic factors to heritability of stimulant drug abuse.
