Participants were from the FinnTwin12 study, a population-based cohort tracking the behavioral development and health habits among all Finnish twins born between 1983 and 1987 (Online Resource 1) [24–26]. Families with twins were identified through the Finnish Central Population Registry with twin pairs enrolled between the ages of 11 and 12. The initial enrollment response rate was 87% (N = 5600 twins) and remained between 85 and 90% for all data collection waves. Questionnaires were collected from individual twins at tightly controlled age-bands in pre- and early adolescence [ages 11–12, 14 and 17; means (standard deviations (SD)]: 11.42 (0.30), 14.05 (0.08), 17.63 (0.26), respectively). Additionally, parents and a teacher of each twin at age 12 completed questionnaires regarding the twins, as well as a teacher of each twin at age 14.
