1. Department of Economics, Columbia University, International Affairs Building, 420 W 118 Street, New York, NY 10027 (e-mail: )
2. School of Management, Yale University, 135 Prospect Street, Box 208200, New Haven, CT 06520, and NBER (e-mail: )
3. Department of Economics, Massachusetts Institute of Technology, 50 Memorial Drive, Building E52, Cambridge, MA 02142, and NBER (e-mail: )
4. Department of Economics, University of Pennsylvania, 428 McNeil Building, 3718 Locust Walk, Philadelphia, PA, 19104, and NBER (e-mail: )