Funding for this research was provided by:
National Institutes of Health (R01-AG023627)
National Natural Science Foundation of China (71673313, 71110107025, 71173227, 71233001)
National Office for Philosophy and Social Sciences (13CJY028)
Training Program for Major Fundamental Research of Central University of Finance and Economics (14ZZD001)
Received: 19 January 2016
Accepted: 6 June 2017
First Online: 1 July 2017
Compliance with ethical standards
: The collection of the data used in this study was supported by NIH R01 grant (R01-AG023627) and the United Nations Fund for Population Activities. This study was supported by the National Natural Science Foundation of China (Grant No. 71110107025, No. 71173227, No. 71673313, and No. 71233001), the National Social Science Fund of China (Grant No. 13CJY028), and Training Program for Major Fundamental Research of Central University of Finance and Economics (Grant No. 14ZZD001).
: The authors declare that they have no conflict of interest.