Psychophysiological measures, particularly electroencephalography (EEG) based measures of neural activity, represent, in part, endogenous, biological attributes whose measurement relies neither on clinical diagnosis nor manifest symptoms of a disorder. EEG-based indices of both stimulus-evoked, task-related brain activity (namely, the P3 event-related potential [ERP] and its associated time-frequency [TF] components) and intrinsic, resting state activity (specifically, activity in the beta frequency band) have been associated with EXT spectrum disorders (e.g. Iacono, Carlson, Malone, & McGue, 2002; Porjesz et al., 2005), and have met criteria (e.g. Frederick & Iacono, 2006; Iacono, 1998) proposed to be necessary for status as an endophenotype: 1) heritability (van Beijsterveldt, Molenaar, de Geus, & Boomsma, 1996; van Beijsterveldt & van Baal, 2002; Yoon, Iacono, Malone, & McGue, 2006), 2) association with multiple disorders in the EXT spectrum (Gilmore, Malone, Bernat, & Iacono, in press; Iacono et al., 2002; Porjesz et al., 2005; Rangaswamy et al., 2002), 3) presence in unaffected relatives of those with a disorder (Begleiter, Porjesz, Bihari, & Kissin, 1984; Hill, Muka, Steinhauer, & Locke, 1995; Rangaswamy et al., 2004), and 4) in currently unaffected
