Endophenotypes are laboratory-based measures of endogenous characteristics of an individual that reflect genetic risk for a specific disorder or spectrum of disorders. One essential criterion for an endophenotype is strong psychometric properties, such as reliability of measurement, and existing research indicates that EEG parameters qualify. Split-half reliability estimates have been found uniformly to approach 1 for measures of EEG power in different frequency ranges (Kondacs & Szabo, 1999), a measure of the energy in a signal, and within-session correlations are relatively robust to the amount of contamination by eye movement activity (Gasser, Bacher, & Steinberg, 1985). Test-retest stability estimates are typically greater than .80 over two- to four-week (Gudmundsson, Runarsson, Sigurdsson, Eiriksdottir, & Johnsen, 2007; Salinsky, Oken, & Morehead, 1991), and have been reported to exceed .70, at least for alpha, beta, and theta power, over two to five years (Kondacs & Szabo, 1999). Stability estimates for delta power are typically smaller (Gasser et al., 1985; Kondacs & Szabo, 1999; Pollock, Schneider, & Lyness, 1991). In addition, individual differences in developmental trajectories have been reported that distinguish children with autism
