Given our interest in studying past-year alcohol use, we excluded from the current analyses individuals who reported that they abstained from alcohol use in the year prior to the interview (FF4 past-year abstainers: N = 785, 36.2%; MF2 past-year abstainers: N = 708, 24.4%). Individual-level analyses were based on 1,382 female twins in FF4 and 2,218 male twins in MF2. FF4 twin-pair analyses were based on data from 480 complete twin pairs (294 monozygotic [MZ] pairs and 186 dizygotic [DZ] pairs) and 417 twins (258 MZ and 159 DZ) without cotwin participation. One twin pair of unknown zygosity and 3 individuals from triplets were excluded from FF4 twin-level analyses. MF2 twin-pair analyses were based on 763 complete pairs (465 MZ pairs and 298 DZ pairs) and 686 twins without cotwin participation (459 MZ and 227 DZ); 6 individuals from triplets were excluded. Zygosity classification is estimated to be >95% accurate (Kendler and Prescott, 2006). Demographic characteristics are summarized in Table 1.
