Catherine H. Mercer and Andrew J. Copas are with the Centre for Sexual Health and HIV Research at University College London, London, England. At the time of the study, Julia V. Bailey was with Kings College London, London. Anne M. Johnson is with the Department of Primary Care and Population Sciences at University College London, London. Bob Erens is with the National Centre for Social Research, London. Kaye Wellings is with the London School of Hygiene and Tropical Medicine, London. Kevin A. Fenton is...