1. Department of Economics, Stanford University, 579 Serra Mall, Stanford, CA 94305 and National Bureau of Economic Research (NBER) (e-mail: )
2. Department of Economics, Massachusetts Institute of Technoloyg (MIT), 50 Memorial Drive, Cambridge, MA 02142 and NBER (e-mail: )
3. Department of Economics, MIT, 50 Memorial Drive, Cambridge, MA 02142 (e-mail: )
4. Vancouver School of Economics, The University of British Columbia, 6000 Iona Drive, Vancouver, BC, Canada, V6T1L4 (e-mail: )