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 genetically simpler than clinical diagnostic categories, and that are less reliant on manifest behavior for diagnosing affected individuals (Gottesman & Gould, 2003; Iacono, 1998). Such traits, known as endophenotypes, are putatively less complex correlates of the disorder that, because they tap the neurobiological basis of gene effects, are closer to gene action, and can be powerful aids in discovering the disorder’s genetic etiology.
