FinnTwin16 (FT16) is a population-based study consisting of 5 consecutive birth cohorts of Finnish twins. All twins were identified through Finland’s Population Register Center, permitting exhaustive and unbiased ascertainment. Zygosity was determined using a well-validated questionnaire completed by both co-twins at the baseline, as described elsewhere (Kaprio et al., 1991). FT16 consists of twins born 1975 to 1979 (Kaprio et al., 2002). The 5 birth cohorts contained 3,065 families of twins in which both twins were living and residing in Finland at the age of 16. Details about data collection have previously been published (Kaprio, 2006; Kaprio et al., 2002). Briefly, 4 waves of postal questionnaires were completed at ages 16, 17, 18.5, and as young adults. Here, we analyze data from the most recent questionnaire and focus on AC and alcohol problems in adulthood. The average age for the respondent twins at this assessment was 24.4 years (SD = 1.50, range 22.8 to 27.2), with a response rate of 88.1%. For ease of presentation, this assessment is referred to as age 25 throughout this paper. Parallel to current practice
