1. Department of Economics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, E52-446, Cambridge, MA 02142 (e-mail: )
2. Laboratory for Information and Decision Systems, Massachusetts Institute of Technology, 77 Massachusetts Avenue, 32-D608, Cambridge, MA 02139 (e-mail: )
3. Columbia Business School, Columbia University, 3022 Broadway, Uris Hall 418, New York, NY 10027 (e-mail: )