[1] Univ London Imperial Coll Sci Technol & Med, Dept Epidemiol & Publ Hlth, London W2 1PG, England
[2] Kings Coll London, Twin Res Unit, London SE1 7EH, England
[3] Univ London London Sch Hyg & Trop Med, Dept Epidemiol & Populat Hlth, London WC1E 7HT, England
[4] Univ London Imperial Coll Sci Technol & Med, Div Neurosci & Mental Hlth, London SW7 2AZ, England