Participants were part of the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders (VATSPSUD), which has been previously described (Kendler and Prescott, 2006). The current sample includes same-sex female twins from the fourth wave of data collection for that subsample (N=1910; 1137 members of monozygotic [MZ] pairs; 773 members of dizygotic [DZ] pairs), and same-sex male twins and opposite sex twins from the second wave of data collection for that subsample (N=5544; 1643 members of male MZ pairs, 1250 members of male DZ pairs, and 2651 members of opposite-sex pairs). The sample is 56% male. Consistent with prior reports using this data (Kendler and Prescott, 2006), the modest number of lifetime abstainers from alcohol in this sample were considered to be unaffected.
