Because drug abusers consume a wide variety of psychoactive substances with substantially different pharmacological properties involving distinct molecular targets in the brain (Koob and Le Moal, 2006), it is of substantial interest to determine the degree to which genetic and environmental risk factors for these substances are specific to individual pharmacological classes or non-specific in predisposing to dependence on a range of psychoactive drugs (Goldman and Bergen, 1998; Tsuang et al., 1998).
