Twin studies provide strong evidence of shared genetic liability across a number of different psychiatric conditions. For example, in a large study of the genetic architecture of the major common psychiatric and substance use disorders, Kendler et al. [2003], using data from the Virginia Twin Registry, demonstrated that there were two broad genetic factors: one that contributed to externalizing disorders (alcohol dependence, drug abuse/dependence, childhood conduct disorder, and adult antisocial behavior) and a second that contributed to internalizing disorders (major depression, generalized anxiety disorder and phobia). These findings have implications for gene identification efforts, as they suggest that some genes may not be specific to any one disorder, but rather, may predispose to a variety of psychiatric outcomes. Furthermore, individuals meeting a psychiatric diagnosis are often a heterogeneous group clinically. For example, in the case of alcohol dependence, affected individuals often vary on a number of important dimensions, including age of onset, course of illness, and the presence of comorbid conditions [Cloninger, 1987; Babor et al., 1992; Hesselbrock and Hessel-brock, 1994; Finn et al., 1997]. Evidence from twin and family
