1 Department of Social and Environmental Health Research, London School of Hygiene and Tropical Medicine, London WC1H 9SH, UK ellen.flint@lshtm.ac.uk.
2 Department of Social and Environmental Health Research, London School of Hygiene and Tropical Medicine, London WC1H 9SH, UK.
3 ESRC International Centre for Lifecourse Studies in Society and Health, Research Department of Epidemiology and Public Health, University College London, London WC1E 6BT, UK.