[1] Renmin Univ China, Sch Agr Econ & Rural Dev, Beijing, Peoples R China
[2] Natl Univ Singapore, Dept Psychol Med, Singapore 117548, Singapore
[3] Natl Univ Singapore, Dept Stat & Appl Probabil, Singapore 117548, Singapore
[4] Duke Univ, Ctr Study Aging & Human Dev, Durham, NC USA
[5] Duke Univ, Sch Med, Div Geriatr, Durham, NC USA
[6] Peking Univ, Natl Sch Dev, Ctr Hlth Aging & Dev Studies, Beijing 100871, Peoples R China