1. Department of Economics, Massachusetts Institute of Technology, 77 Massachusetts Avenue E18-218, Cambridge, MA 02139, and NBER (e-mail: )
2. Department of Economics, Bocconi University, via Roentgen 1, 20136 Milan, Italy, and IGIER (e-mail: )
3. Department of Economics, Columbia University, International Affairs Building, 420 West 118th Street, New York, NY 10027, and NBER (e-mail: )