1. Harvard University, 1050 Massachusetts Avenue, Cambridge, MA 02138, and National Bureau of Economic Research.
2. Department of Economics, Yale University, 77 Prospect Street, New Haven, CT 06520.
3. Department of Economics, Harvard University, Lit-tauer Center, Cambridge, MA 02138, and NBER.
4. Department of Economics, Wesleyan University, PAC 123, 238 Church Street, Middletown, CT 06459.
5. Department of Economics, University of California, Berkeley, Berkeley, CA 94720.