Several variants in the alcohol metabolism genes (i.e., ADH1B) and nicotine metabolism genes (i.e., CYP2A6) have low frequencies (1%–5% minor allele frequency) and generally reduce risk for dependence, suggesting that human populations might be genetically predisposed to develop addiction, with rare variant alleles leading to reduced risk. This is supported in the nicotine literature, in which most people who smoke develop some symptoms of dependence, whereas only 20% smoke without developing any symptoms of dependence (14). It could be that as a result of some unknown evolutionary selection pressure, most people are predisposed to addiction when exposed to substances.
