Efforts to discover genes underlying EXT spectrum disorders have reported significant results implicating genes for alcohol dependence and related phenotypes (e.g. ADH, GABRA2, GABRR1, GABRR2; Bice et al., 2008; Covault, Gelernter, Jensen, Anton, & Kranzler, 2008; Dick et al., 2006; Edenberg et al., 2004; Schuckit, 2009; Wall, Shea, Luczak, Cook, & Carr, 2005; Xuei et al., 2009), other substance use disorders (Agrawal et al., 2006; Hopfer et al., 2007; Stallings et al., 2003), and behavioral disorders (Amin et al., 2009; Burt & Mikolajewski, 2008; McGough et al., 2008; Rommelse et al., 2008). These findings, however, are tempered by the fact that psychopathological disorders are etiologically complex and multifactorial in origin, reflecting the combined influence and interactions of genetic, epigenetic, stochastic, and nongenetic factors (Cowan, Kopnisky, & Hyman, 2002; Gottesman & Gould, 2003; Schork, Greenwood, & Braff, 2007). This complexity in etiology, coupled with an incomplete understanding of how best to identify and diagnose psychological disorders (e.g. Krueger, Watson, & Barlow, 2005), has hampered the search for psychopathology-related genes. This genetic and phenotypic complexity belies the need for phenotypes that are
