1. Department of Economics, Stanford University, 579 Serra Mall, Stanford, CA 94305-6072, and National Bureau of Economic Research (NBER) (e-mail: )
2. Department of Economics, Massachusetts Insitute of Technology, 50 Memorial Drive, Cambridge, MA 02142, and NBER (e-mail: )
3. Department of Economics, Massachusetts Institute of Technology, 50 Memorial Drive, Cambridge, MA 02142, and NBER (e-mail: )