Launched in the 1990s and still ongoing, the Minnesota Twin Family Study (MTFS) is a longitudinal investigation of twin children and their parents (W. G. Iacono, 1998; W. G. Iacono & McGue, 2002; W. G. Iacono, McGue, & Krueger, 2006). Eligible families were identified using publicly available birth certificates indicating that they had twin children born in the state of Minnesota. All families with same-sex twins meeting basic inclusion criteria were considered eligible. Over 80% of eligible families participated, yielding a sample that was broadly representative of Minnesota families with children living at home based on the US Census of 2000. Parents and their offspring have been evaluated using psychophysiological paradigms selected for their potential to yield endophenotypes for substance use disorders, antisocial personality disorder, schizophrenia, and mood disorders (W. G. Iacono, 1998). Using data from 4905 of these participants, constituting the largest sample ever used for this purpose, we carried out in parallel a series of seven investigations designed to identify molecular genetic variants associated with 17 psychophysiological variables involving EEG frequency-based measures, P300 oddball visual event-related potential, antisaccade
