1. Department of Sociology, University of Oxford, Oxford, OX3 1UQ, UK
2. Department of Medicine, Stanford University, Palo Alto, CA, 94305, USA
3. Department of Public Health and Policy, LSHTM, London, WC1H 9SH, UK
4. Department of Epidemiology and Public Health, University College London, London, WC1E 6BT, UK