With regard to psychological disorders, an endophenotype is a measurable trait intermediate between the clinical manifestation of the disorder and the genes underlying the disorder (Gottesman & Gould, 2003). The endophenotype, then, is putatively a less complex correlate of the disorder that is closer to gene action, and that can aid in discovering the disorder’s genetic etiology. While P3-AR has been associated with a general vulnerability to externalizing, its utility as an endophenotype has only been tested with regard to alcohol use disorders. Visual P3 amplitude itself has shown significant linkage on chromosomes 2, 5, 6, 13, and 17 (Begleiter et al., 1998; Porjesz et al., 2002; Porjesz et al., 2005), and an association between P3 amplitude and the dopamine receptor A1 allele has been found (Hill et al., 1998). With respect to alcohol use, there is evidence that particular genetic loci affecting P3 amplitude also influence risk of alcohol dependence. Williams et al. (1999) found that reduced P3 amplitude and an alcoholism diagnosis were jointly linked to a region on chromosome 4 near the alcohol dehydrogenase gene (ADH3), and
