1. Department of Economics, University of California-Riverside, 3128 Sproul Hall, Riverside, CA 92521 (e-mail: )
2. Department of Economics and Courant Institute of Applied Mathematics, New York University, 19 West Fourth Street, New York, NY 10003 (e-mail: )
3. Department of Economics, Johns Hopkins University, Wyman Park Building 592, 3100 Wyman Park Drive, Baltimore, MD 21211, and Department of Economics, National University of Singapore (e-mail: )