1. From the Division of Community Health Sciences, St George's, University of London, London, United Kingdom (CGO, PHW, SJK, DGC, ARR, and DPS); the Department of Social Medicine, University of Bristol, Bristol, United Kingdom (RMM and GDS); Public Health and Clinical Medicine, Umea University, Umea, Sweden (EB); the MRC National Survey of Health and Development, Department of Epidemiology and Publi