1. Department of Economics, Massachusetts Institute of Technology, 50 Memorial Drive, E52-371, Cambridge, MA 02142 and National Bureau of Economic Research (NBER) (e-mail: )
2. Department of Economics, Stanford University, 579 Serra Mall, Stanford, CA 94305 and NBER (e-mail: )
3. Department of Social Sciences, United States Military Academy, 607 Cullum Road, West Point, NY 10996 (e-mail: )