[1] Karolinska Inst, Dept Publ Hlth Sci, Child & Adolescent Publ Hlth Epidemiol Grp, SE-17176 Stockholm, Sweden
[2] Univ Bristol, Dept Social Med, Bristol BS8 1TH, Avon, England
[3] Univ London London Sch Hyg & Trop Med, Dept Epidemiol & Populat Hlth, London WC1E 7HT, England
[4] Stockholm Ctr Publ Hlth, Div Epidemiol, Stockholm, Sweden