1. United Nations Population Division, New York, NY, USA
2. Virginia Polytechnic Institute and State University, Blacksburg, USA
3. Vanderbilt University, Nashville, TN, USA
4. Division of Geriatrics, Department of Medicine, Duke University Medicial Center, Durham, NC, USA
5. Physical Medicine & Rehabilitation Service, Durham Veteran Affairs Medical Center, Durham, NC, USA
6. Center for the Study of Aging and Human Development and Geriatrics Division, Duke University, Durham, NC, USA
7. National School of Development, Peking University, Beijing, China