Evidence from twin studies supports the idea that at least 40% of the addictions to alcohol, tobacco, and other drugs have genetic influences (Merikangas 1998; Tsuang et al 1998; McGue 1999; Karkowski et al 2000; Theodore et al 2003; Uhl and Grow 2004). For example, Jacob et al (2003) reported that offspring of monozygotic and dizygotic twins with a history of alcohol dependence were found to exhibit alcohol abuse or addiction more frequently than offspring of nonalcoholic fathers, and offspring of an alcohol-abusing monozygotic twin whose co-twin was alcohol dependent were more likely to be alcohol-dependent than offspring of nonalcoholic twins. However, in the absence of paternal alcoholism, offspring with high genetic risk (the unaffected father’s co-twin is alcoholic) showed lower rates of alcoholism than children of alcoholics (Jacob et al 2003). Genome scans have identified multiple addiction vulnerability loci, but no regions that seem to contain genes of major effect in alcoholics or poly-substance abusers (Long et al 1998; Foroud and Li 1999; Uhl et al 2001).
