limiting initial use. Family, twin, and adoption studies also convincingly demonstrate a substantial genetic contribution to the development of addiction to nicotine, alcohol, and illicit drugs. Heritability estimates for nicotine, alcohol, and drug addiction are in the range of 50% to 60% (Heath et al., 1997; Tsuang et al., 1998; Kendler et al., 2003; Li, 2006). In general, it appears that environmental factors have a stronger effect on initiation, whereas genetic factors play a larger role in the transition from regular use to the development of addiction (Vink et al., 2005). Given the robust behavioral evidence for the role of genetic influence in addiction, genetic studies are warranted.
