EEG signals were subjected offline to a 7.5-Hz low-pass digital filter to attenuate high-frequency noise and to the blink-correction algorithm of Gratton, Coles, and Donchin (1983). As in past studies from the MTFS that used this task (e.g., Carlson & Iacono, 2006; Carlson, Iacono, & McGue, 2002, Carlson, Iacono, & McGue, 2004; Carlson, Katsanis, Iacono, & Mertz, 1999; Carlson et al., 2007; Hicks et al., 2007; Iacono, Carlson, Malone, & McGue, 2002; Katsanis, Iacono, McGue, & Carlson, 1997; Patrick et al., 2006), a rater unaware of participant diagnosis or family history used an interactive computer algorithm to identify P300 at the Pz site. The algorithm identified the maximum voltage in a window from 200 to 800 ms after stimulus onset. This liberal latency range was chosen so as not to artificially constrain P300 latency. The minimum and maximum observed latencies in this study were 300.78 and 617.19 ms, respectively. In cases in which there was more than one peak, the rater chose the second peak. The peak selected was also checked against the waveforms from P3 and P4 and corrected if needed. Amplitude was the difference in voltage between the mean of the 500-ms baseline and the selected peak.
