1. Centre for Longitudinal Studies, Department of Social Science, UCL Institute of Education, University College London, London, England
2. Department of Epidemiology and Public Health, University College London, London, England
3. School of Biological and Population Health Sciences, Oregon State University, Corvallis
4. MRC Unit for Lifelong Health and Ageing, Department of Population Science and Experimental Medicine, University College London, London, England