Once it is established that the affected index case in a genetic study has the endophenotype, that characteristic should be present in other affected family members because they presumably share the same genetic liability. Although SUDs are not expressed in everyone at genetic risk, many unaffected first degree relatives of a proband, inclulding offspring, will share genetic liability with the proband, and they should also possess the attribute at elevated rates. Finally, the endophenotype should show diagnostic specificity. Importantly, however, given the high rates of comorbidity associated with common mental disorders, it is unreasonable to expect the endophenotype to be exclusive to a single disorder. Ideally, it should be exclusive to a spectrum of disorders that are likely to be comorbid because they share etiologic mechanisms, including common genetic determinants (as appears to be the case for SUDs, see e.g., Krueger, et al., 2002). Indeed, an endophenotype may be used as a tool to identify phenotypes that fall into the spectrum (Yoon, Iacono, Malone, & McGue, 2006). Alternatively, to the extent that different disorders share a dysfunctional process in common,
