Fewer studies have examined genetic influences on illicit drug addiction. Heritability of the use/dependence on stimulants, sedatives, and heroin in males is 0.33, 0.27, and 0.54, respectively.10 These values are similar to those seen in females, although such studies are rarer.11 Moreover, twin studies indicate a large overlap between the genetic predispositions to dependence and that to most classes of substances. For example, twin studies suggest that nicotine and alcohol dependencies share more than 60% of their genetic vulnerabilities.12
