FinnTwin12 (FT12) is a longitudinal study of five consecutive birth cohorts (1983-1987) of Finnish twins and their families (Kaprio et al., 2002). Families with twins were identified via Finland’s Population Register Centre and contacted when the twins were 11-12 years. After baseline data collection (N = 2,724 families, response rate 87%), the sample has been followed up with questionnaire surveys at ages 14 years, 17.5 years, and as young adults at an average age of 24 years (SD = 1.7, range 19-27), with consistently high response rates (85-90% at each wave of data collection; Kaprio et al., 2002; Kaprio, 2006). The study protocol was approved by the Ethical Committee of the University of Helsinki and the IRB of Indiana University. Parents provided written informed consent for their and their children’s participation. At baseline, parents also reported on their own socioeconomic and health characteristics, including education, alcohol use and smoking.
