1. Department of Economics, Cornell University, 480 Uris Hall, Ithaca, NY 14853 and National Bureau of Economic Research.
2. S. C. Johnson Graduate School of Management, Cornell University, 324 Sage Hall, Ithaca, NY 14853.
3. Department of Economics, University of Michigan, 312 Lorch Hall, Ann Arbor, MI 48109 and National Bureau of Economic Research.
4. Department of Economics, Cornell University, 445 Uris Hall, Ithaca, NY 14853.