Two large population-based twin studies have explored this question for illicit psychoactive substances (Kendler et al., 2003; Tsuang et al., 1998). Tsuang et al. (1998) examined the abuse of 5 classes of illicit drugs (marijuana, sedatives, stimulants, heroin or opiates, and psychedelics) in 3372 male twin pairs from the Vietnam Era register. They found that most genetic and environmental risk factors were shared amongst these substances, although modest amounts of drug specific genetic and environmental influences were also seen (Tsuang et al., 1998). Kendler et al. (2003) examined the abuse or dependence of six illicit psychoactive drug classes (cannabis, sedatives, stimulants, cocaine, opiates, and hallucinogens) in 1196 male–male twin pairs from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders (VATSPSUD). They found that all the genetic influences on abuse/dependence of these six substance classes were non-specific and shared across substances. In a subsequent confirmatory analysis of abuse/dependence of cannabis, cocaine, alcohol, nicotine, and caffeine in both the male and female twins from VATSPSUD, Kendler et al. (2007) found evidence for highly correlated but separable licit and illicit common factors with modest degrees of substance specific genetic influences.
