1. Massachusetts Institute of Technology Department of Economics, 77 Massachusetts Avenue, Room E17-216, Cambridge, MA 02142 (e-mail: )
2. Wharton School at the University of Pennsylvania, 3620 Locust Walk, 1358 Steinberg Hall—Dietrich Hall, Philadelphia, PA 19104 (e-mail: )
3. MIT Department of Economics, 77 Massachusetts Avenue, Room E17-220, Cambridge, MA 02142 (e-mail: ).