This heritable variation can be parsed into those genetic influences that are specific to each drug and importantly, those genetic factors that confer a general predisposition to alcohol and/or substance use disorders, and even other disinhibited behaviors (Hicks et al., 2013; Krueger et al., 2002). Two large twin studies have convincingly shown that a preponderance of the genetic factors influencing illicit drug use disorders overlap (Kendler et al., 2003; Tsuang et al., 2001). Noticeably, when these models were extended to include alcohol use disorders, there was evidence for highly correlated genetic factors (r=0.82) that individually influenced the covariation in alcohol and nicotine dependence as well as cannabis and cocaine dependence (Kendler et al., 2007). The extent of genetic overlap was strong for some substances - for instance, 55% and 24% of the genetic variance in alcohol dependence was due to the licit and illicit drug factors respectively, with the remainder being substance-specific. In contrast, for nicotine dependence, 63% of the genetic variance was drug-specific (with 26% and 11% attributable to heritable variation in the licit and illicit factors respectively). Similar
