1. Department of Economics, Cornell University, 480 Uris Hall, Ithaca, NY 14853, and NBER (e-mail: )
2. Samuel Curtis 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. Department of Economics, Cornell University, 455 Uris Hall, Ithaca, NY 14853 (e-mail: ).