Institute of Agricultural Economics and Development Chinese Academy of Agricultural Sciences Beijing 100081 P.R.China;
School of Economics Zhejiang University of Finance & Economics Hangzhou 310018 P.R.China;
China Academy for Rural Development Zhejiang University Hangzhou 310058 P.R.China;
China Academy for Rural Development Zhejiang University Hangzhou 310058 P.R.China;
International Food Policy Research Institute Washington D.C. 20005 USA;