1. Department of Economics, Cornell University, 480 Uris Hall, Ithaca, NY 14853, and NBER (e-mail: )
2. S. C. Johnson Graduate School of Management, Cornell University, 324 Sage Hall, Ithaca, NY 14853, and NBER (e-mail: )
3. Department of Economics, University of Michigan, 312 Lorch Hall, Ann Arbor, MI 48109, and NBER (e-mail: )
4. The Wharton School, University of Pennsylvania, 553 Jon M. Huntsman Hall, 3730 Walnut St., Philadelphia, PA 19104 (e-mail: )