Consistent with the alcoholism-P3 literature and our past investigations using this task, only data recorded from the PZ electrode during target trials (N=80) were considered for this report. These trials were averaged together and baseline corrected for each participant. A computer algorithm calculated the amplitude and latency of the largest positive peak between 280 ms and 600 ms after stimulus onset for each participant’s average waveform. Members of twin pairs are the same age and gender, and variability in P300 amplitude related to age or gender can influence apparent twin similarity. Because this may distort the estimates of genetic and environmental influence on a trait (McGue & Bouchard, 1984), we regressed P300 amplitude on age, age2, age × gender, and age2 × gender, and the standardized residuals from these regressions were used in subsequent analyses.
