FinnTwin12 is a population based, longitudinal study that has followed five consecutive birth cohorts of twins born 1983-1987 identified through Finland’s central population registry (n = 5,600 twins and 5,000 of their parents). The study was initially designed to examine genetic and environmental influences on health-related behaviors. Baseline questionnaire data were collected on twins just prior to their 12th birthdays. The twins were followed-up at ages 14.2, 17.5 years, and most recently at 22.2 years, on average (hereafter referred to as 14, 17, and 22 years). Zygosity was determined using a well-validated questionnaire completed by both co-twins and their parent(s) at baseline (Rose et al. 2001) and later by DNA confirmation. Zygosity confirmation by genetic markers at wave four (age 22) revealed that 97% of same-sex pairs retained their original questionnaire zygosity classification (Knaapila et al., 2011).
