1. Department of Social Security, School of Labor and Human Resources , Renmin University of China, 59 Zhongguancun Street, Haidian District, Beijing 100872, China
2. Department of Health Care Policy, Harvard Medical School , 180 Longwood Avenue, Boston, MA 02115, USA
3. Department of Global Health and Social Medicine, Harvard Medical School , 641 Huntington Avenue, Boston, MA 02115, USA