1. University of Michigan Department of Economics, 611 Tappan Street, 318 Lorch Hall, Ann Arbor, MI 48109-1220 and National Bureau of Economic Research.
2. Cornell University Department of Policy Analysis and Management, 103 Martha Van Rensselaer Hall, Ithaca, NY 14853.
3. University of Virginia Department of Economics, Monroe Hall, McCormick Road, Charlottesville, VA 22904 and National Bureau of Economic Research.