1. Department of Economics, MIT, 77 Massachusetts Avenue, E52-406, Cambridge, MA 02139, and NBER (email: )
2. Stanford University, Huang Engineering Center 262, 475 Via Ortega, Stanford, CA 94305 (email: )
3. Wharton, 1455 Steinberg Hall-Dietrich Hall, 3620 Locust Walk, Philadelphia, PA 19104 (email: )
4. Wharton, 1352 Steinberg Hall-Dietrich Hall, 3620 Locust Walk, Philadelphia, PA 19104 (email: )
5. Department of Economics, MIT, 77 Massachusetts Avenue, E52-300, Cambridge, MA 02139 (email: )