FinnTwin12 is a longitudinal twin study launched in 1994 to investigate the developmental epidemiology of health-related behaviors (Rose et al., 2001). From 1994 to 1998, all Finnish families with twins born in 1983–1987 were identified from Finland's Population Register Centre and enrolled into a two-stage sampling design (Kaprio et al., 2002). The first stage included questionnaire assessments of all twins and parents at baseline (87% participation rate, 2,724 families) conducted during the year in which the consecutive twin cohorts reached age 11, with follow-up of all twins at ages of 14 and 17½ years, and as young adults (age 22).
