1. National Economics Research Center and School of Economics, Guangdong University of Finance and Economics, Guangdong, Guangzhou, China
2. School of Nursing, Global Health Institute, and Center for the Study of Aging and Human Development, Duke University, Durham, NC, USA
3. School of Public Health and Sun Yat-sen Center for Migrant Health Policy, Sun Yat-sen University, Guangdong, Guangzhou, China