1. Department of Economics, University of Lausanne and CEPR; Internef, Quartier Chamberonne, CH-1015 Lausanne, Switzerland (email: )
2. Department of Economics, University of Washington, EABCN, CEPR, and NBER; Savery Hall, Box 353330, Seattle, WA 98195 (email: )
3. Department of Economics, Harvard University, CEPR, and NBER; Littauer Center 215, Cambridge, MA 02138 (email: )