Susceptibility to substance abuse disorders aggregates in families, with rates of drug abuse higher among relatives of drug-dependent individuals than in the general population.33,34 There is approximately a 4- to 8-fold increase in risk of substance abuse disorders in individuals with an affected first-degree family member across a wide range of abused substances, including cocaine, alcohol, nicotine, opiates, and cannabis.34-36 The transmission of vulnerability to substance abuse is largely independent of familial aggregation of other psychiatric disorders.35-38 Adoption studies have shown far greater similarities between substance abuse phenotypes with biological relatives than with adoptive family members.39,40 Further, heritability of substance-abuse disorders is significantly higher in monozygotic compared with dizygotic twins.41 Indeed, data from twins suggest that approximately 50% of vulnerability to addiction is heritable.40 Genetic transmission of substance-abuse disorders does not appear to follow simple Mendelian inheritance. In common with other complex diseases, substance-abuse disorders instead appear to depend on the contribution of multiple susceptibility genes that act in concert with epigenetic and environmental factors to increase vulnerability to addiction.39,41-44 Taken together, these observations support a strong genetic component to addiction vulnerability.
