The present sample, the Swedish Twin Study of Child and Adolescent Development, began with all twin pairs born in Sweden between May 1985 and December 1986 (Lichtenstein et al. 2007). Twins were recruited through linkage to the Medical Birth Registry and twins and parents identified were sent study questionnaires (Lichtenstein & Svartengren, 1997). Participants had completed four assessment waves and information from waves 2–4 was used for the present study, when twins were aged 13–14, 16–17 and 19–20 years. Responses rates for each wave were 78, 82 and 59% respectively. The Ethics Committee of Karolinska Institutet, Stockholm, Sweden, approved the questionnaires used. In Sweden, responding to the questionnaire constitutes consent.
