Tsuang et al. [57] conducted a twin study including 3372 male twin pairs from the Vietnam Era Twin (VET) Registry, 1874 of which were monozygotic (MZ) and 1498 of which were dizygotic (DZ), and found that DSM-III-R characterized drug dependence was apparent in 9.5% of the studied population. This study also investigated the co-occurrence of the abuse of drugs belonging to different categories (i.e. marijuana, stimulants, sedatives, heroin, and psychedelics) within an individual as well as their co-twin in order to determine how genetic and environmental factors contributed to concurrent drug abuse behaviour [57]. Using the common vulnerability model, 31% of the variance in drug abuse behaviour was attributed to additive genetic effects (A), while 25% and 44% were attributed to family (C) and non-family (E) environmental factors, respectively [57]. Single independent models of drug abuse patterns show that marijuana abuse was the only category to be influenced by family environmental factors, and heroin abuse had the largest contribution from specific genetic factors that were unshared with other drugs [57].
