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 to the individual heritability of each substance, there is growing evidence that the heritable covariation across substances changes across development (Vrieze et al., 2012; Young et al., 2006). Irrespective of development and substance-specific variation, there is broad consensus that gene discovery efforts targeting aggregate genetic variation that indexes a shared liability to a variety of substance use disorders, as well as disinhibition, can be profitable (McGue et al., 2013; Vrieze et al., 2013a), with one study showing evidence for genomewide pleiotropic effects across substance use disorders (Vrieze et al., 2013b).
