The sample for the current study was comprised of 100 monozygotic (MZ) and 131 dizygotic (DZ) twin pairs, all of whom were 21 years of age or older at the time of last interview. (Age 21 was chosen as the cut-off based on Kaplan–Meier estimations of survival curves for age at onset of each of the three substances.) Of the 255 African-American twin pairs who met age eligibility criteria, 90.6% had complete data for both twins on initiation of use for all substances, resulting in a sample size of 462 twins (231 pairs) for the current study. In the majority of cases (89.4%), data were drawn from the fifth wave of data collection. For the remaining participants, who did not participate in Wave 5 interviews but were 21 years of age or older at the time of Wave 4 data collection, data were drawn from Wave 4 interviews. Mean age at the time of last interview was 25.1 (range = 21–31) years.
