基金项目: | The authors thank the National Science Foundation of China (No. 70673072) and the National Social Science Foundation of China (No.10JZD013) for financial support. We are grateful for helpful discussions with and comments from Professor Ping Wang from Washington University in St. Louis, participants at the 2010 Hong Kong Economic Association Conference and seminar at the Institute for Advanced Study, Wuhan University on earlier versions of the present paper. |