Although a case could be made that items in Table 1 designated as “desirable” also constitute essential criteria, absent information about the etiology of the disorder and/or the endophenotype, it would be difficult to determine whether these criteria are in fact met. Compared to a diagnosis defined by self-reported behavior problems, an endophenotype is expected to be more strongly linked to the effects of specific genes governing brain processes, but the genes and the processes they govern must be known before this can be determined with certainty. Although we do not yet have this detailed knowledge for P3AR, we are beginning to uncover it for other brain-derived endophenotypes, providing proof in principle that endophenotypes may provide substantive etiological insights (Erk, et al., 2010; Hodgkinson, et al., 2010; Roeske, et al., 2009). In addition, many studies have reported associations between P300 amplitude and specific genes or chromosomal loci (Begleiter, et al., 1998; Chen, et al., 2010; Comings, Dietz, Johnson, & MacMurray, 1999; Hill, et al., 1998; Johnson, et al., 1997; Porjesz & Rangaswamy, 2007; Williams, et al., 1999; Wright, et al.,
