1. Center for Chinese Agricultural Policy, School of Advanced Agriculture; Peking University; Yiheyuan Road No. 5, Haidian District Beijing 100871 China
2. Center for Chinese Agricultural Policy, Institute of Geographic Sciences and Natural Resources Research; Chinese Academy of Sciences; Datun road No.11a, Chaoyang District Beijing 100101 China
3. Development Research Group; World Bank; 1818 H St. NW Washington DC 20433 USA