1. Center for the Study of Aging and Human Development and Geriatric Division of Medical School, Duke University, Box 3003, Room 1506, BUSSE Building, Durham, NC 27710, USA
2. China Center for Economic Research, National School of Development at Peking University, Beijing 10087, China
3. Max Planck Institute for Demographic Research, Konrad Zuse Straße 1, 18057 Rostock, Germany