The three alcohol use outcomes loaded between 0.889 and 0.973 on the largest factor (accounting for 88.2% of the total variance), supporting the use of a single composite variable as a measure of AAU. Prior to transformation, all three variables were winsorized down to the 95th percentile, where the top 5% of extreme values were replaced with the value at the 95th percentile. This was done to reduce skew but preserve the meaningfulness of extreme alcohol use behavior. The 95th percentile was 100 for NumIntox, 24 for MaxCon, and 12 for HeavyAve. The transformed alcohol use variables were also subjected to age and age2 correction (McGue & Bouchard, 1984). The overall AAU factor score distribution was standardized to a mean of 0 and a standard deviation of 1. Larger factor scores indicated greater alcohol use; zero reflected the sample’s mean factor score. Descriptive statistics for the three alcohol use variables (winsorized but untransformed) are provided in Table 1. 973 participants (312 male MZs; 313 female DZs; 166 male DZs; 182 female DZs) had P300 amplitude data available for inclusion in this study. This sample was comprised of members of 321 male twin pairs (216 MZ pairs) and 345 female twin
