The study sample consisted of identical (monozygotic; MZ) and non-identical (dizygotic; DZ) Finnish twins, participating in the FinnTwin12 (FT12) study. FT12 is a population-based twin study that consists of five consecutive birth cohorts (1983–1987) [22]. The Central Population Registry was used to identify families with twins. A total of 2724 families (87% of all identified eligible families) returned the family questionnaire, and from these families, 2567 twin pairs completed the baseline questionnaires [23].
