1. Department of Economics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building E52-528, Cambridge, MA 02139 (e-mail: )
2. Department of Economics, Harvard University, Littauer 318, 1805 Cambridge Street, Cambridge, MA 02138 (e-mail: )
3. Department of Economics, University of California-Berkeley, 697D Evans Hall, #3880, Berkeley, CA 94720 (e-mail: )