1. Center for Economic and Social Research, University of Southern California, 635 Downey Way, Los Angeles, CA 90089-3332 (e-mail: )
2. Graduate School of Business, Columbia University, Uris Hall, 3022 Broadway, New York, NY 10027 (e-mail: )
3. Dietrich School of Arts and Sciences, Department of Economics, University of Pittsburgh, 4901 Wesley W. Posvar Hall, 230 South Bouquet Street, Pittsburgh, PA 15260 (e-mail: )