1 Center for the Study of Aging and Human Development, Medical School of Duke University, Durham, NC, 27710, USA; Center for Healthy Aging and Development Studies, National School of Development, Raissun Institute for Advanced Studies, Peking University, Beijing, China. Electronic address: zengyi@nsd.pku.edu.cn.
2 Department of Sociology, Centre for Family and Population Research, National University of Singapore, Singapore.
3 Institute for Global Health, University College London, London, UK; Institute for Global Health, School of Public Health, Zhejiang University, Zhejiang Sheng, China.
4 Danish Aging Research Centre, Unit of Epidemiology, Biostatistics and Biodemography, Institute of Public Health, University of Southern Denmark, Odense, Denmark.
5 Max Planck Institute for Demographic Research, Rostock, Germany.