These dynamic shifts in the importance of genetic effects become even more complex when viewed in the context of another important principle about genetic effects: genetic influences on an outcome can operate through multiple pathways. In the case of alcohol use, twin studies indicate that much of the predisposition to alcohol problems is not unique to alcohol problems at all – but rather is shared with a number of other psychiatric conditions that fall under the general spectrum of externalizing behavior, including other forms of drug dependence, adult antisocial behavior, and childhood conduct disorder, as well as personality traits related to impulsivity and behavioral disinhibition (Kendler et al., 2003; Krueger et al., 2002; Young et al., 2000). In fact, data from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders indicate that as much as 65% of the genetic influences on alcohol dependence is shared with these other disorders, with only ~35% of the heritability being genes that are specific to alcohol dependence (Kendler et al., 2003). These latter, more specific influences are likely to include genes that
