Marian Pitts1, Martin Holt2, Catherine H Mercer3. 1. Australian Research Centre in Sex, Health and Society, La Trobe University, 215 Franklin Street, Melbourne, Vic. 3000, Australia. 2. Centre for Social Research in Health, University of New South Wales, John Goodsell Building, Sydney, NSW 2052, Australia. 3. Research Department of Infection & Population Health, University College London, Mortimer Market Centre off Capper Street London, London WC1E 6JB, UK.