1. University of California, Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, and NBER (email: )
2. Department of Economics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, and NBER (email: )
3. Department of Economics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139 (email: )
4. Department of Economics, Northwestern University, 2211 Campus Drive, Evanston, IL 60208, and NBER (email: )