Vulnerability to addictions is a complex trait with substantial genetic influences that are documented by data from family, adoption and twin studies [1]–[4]. Twin studies also document shared heritable influences on vulnerability to dependence on addictive substances from different pharmacological classes (eg nicotine and stimulants) [2], [3], [5]. In individuals from most populations, each gene's variants are likely to contribute modestly to substance dependence vulnerability. Contributions of nicotinic receptor gene variants to individual differences in smoking quantity [6]–[10] and acetaldehyde dehydrogenase/alcohol dehydrogenase variants to risk for alcohol dependence in Asians may provide larger effects of variants at single loci [11], [12]. However, combined data from linkage and initial genome wide association studies (GWA) [6], [13]–[17] suggest that most genetic effects on vulnerability to substance dependence are likely to be polygenic.
