1. Department of Economics, McNeil Blg. 160, University of Pennsylvania, Philadelphia, PA 19104-6297.
2. Research Department, The Federal Reserve Bank of Richmond, 701 East Byrd Street, 22nd Floor, Richmond, VA 23261, and Federal Reserve Bank of St. Louis.
3. School of Economics, Fudan University, Shanghai, China 200433 and Department of Economics, Iowa State University, Ames, IA 50011.