1. Leverhulme Centre for Demographic Science, Department of Sociology, and Nuffield College, University of Oxford, Oxford, OX1 1JD, United Kingdom
2. Department of Population Health, London School of Hygiene and Tropical Medicine, London, WC1E 7HT, United Kingdom
3. Interdisciplinary Centre on Population Dynamics, University of Southern Denmark; Odense 5000, Denmark
4. University of Colorado Population Center, Institute of Behavioral Science, University of Colorado Boulder, Boulder, CO 80302