Using diagnoses as the sole phenotypes may not be optimal for genetic dissection of complex (non-Mendelian) psychiatric diseases. Psychiatric diagnoses are dichotomous; either an individual is affected or unaffected (Begleiter and Porjesz 2006; Gottesman and Gould 2003), with diagnoses based on symptomatic definitions that reflect much heterogeneity in the diseases. In addition, psychiatric diagnoses may adjust over time due to the changes in the diagnostic criteria. In recent decades, research has been directed at identifying the characteristic traits (i.e., endophenotypes or intermediate phenotypes) in subjects with psychiatric diseases as well as their relatives in order to understand the factors underlying the pathogenesis of the disorders. It is believed that these endophenotypes are closer to gene action than diagnostic categories, and that they provide a more powerful strategy in searching for genes involved in producing psychiatric diagnoses (Almasy and Blangero 1998; Gottesman and Gould 2003; Porjesz and others 2005).
