FinnTwin12 (FT12) is a population-based, developmental twin study of health-related behaviors and correlated risk factors (Kaprio et al., 2002). It consists of 5 consecutive birth cohorts (1983–1987) of twins identified in Finland’s Population Registry Center (PRC), permitting exhaustive and unbiased ascertainment of all twins living and resident in the country. Questionnaires were mailed to all eligible families, of which 87% (n = 2,724 families) completed the initial family questionnaire. Noncompliance at these initial stages was associated neither with family structure, area of residence within Finland, parental age, nor sex or zygosity of the adolescent twin pairs. Immediately on receipt of the completed family questionnaire, individual questionnaires were mailed to both co-twins and both their parents (including parents not residing with either twin child). The twins’ self-report questionnaires were mailed in the late autumn of the year in which the consecutive birth cohorts reached age 11, and most twins (89.4%) returned their questionnaires before the end of that year (mean age at response 11.4 years, SD = 0.3). We obtained teacher ratings during winter/spring of the year each cohort turned age
