Families in FinnTwin12 were identified from Finland’s Population Registry, permitting comprehensive nationwide ascertainment for twins born from 1983 to 1987. Baseline collection occurred when twins were aged approximately 11–12 years, with a sample of approximately 5600 twins (87% participation) and their families [36]. Follow-up surveys occurred at ages 14, 17.5, and during young adulthood (age range = 20–26). Twin zygosity was determined using items developed for twin children [42]. Confirmation by multiple genetic markers revealed that 97% of same-sex pairs retained the original questionnaire-based zygosity classification [43]. The Helsinki University Central Hospital District’s Ethical Committee and Indiana University’s Institutional Review Board approved the FinnTwin12 study. Of those in the larger sample, a subset of intensively studied individuals also received in-depth clinical interviews (n = 1347) and participated in DNA collection as young adults. In the present study, we limited our analyses to those who had complete information on all relevant study variables and who had initiated alcohol use (n = 1201). The analytical subset did not differ significantly from the full sample in terms of demographic characteristics or alcohol misuse (see Supporting information, Table S1 for more detail).
