stages of pregnancy, or handicapped (N = 18); or deceased (N = 1). Of the 436 eligible pairs successfully contacted and invited to participate, 300 (68.8%) did so. The first 177 of these normally hearing twin pairs (plus individual twins from 4 twin pairs in which only 1 co-twin contributed ERP data) completed a full-day study protocol including EEG/ERP assessments; these 177 twin pairs (46 monozygotic [MZ], 131 dizygotic [DZ], including 49 brother-sister twin pairs) and individual twins from 4 other twin pairs contributed data for the results we here report. These 358 individual twins (187 females, 171 males) ranged in age from 23 to 28 (mean age = 25.8, SD = 1.0) at the time of ERP assessments. Travel and overnight accommodation were provided for twins coming from a distance, and all participant twins were paid a modest incentive. All study protocols were approved by the IRB at Indiana University and by the Ethics Committee of the Helsinki metropolitan hospital district.
