1. MIT, 50 Memorial Drive, Cambridge, MA 02142, and NBER (e-mail: )
2. Northwestern, 2040 Sheridan Road, Evanston, IL 60208, and NBER (e-mail: )
3. Northwestern, 2040 Sheridan Road, Evanston, IL 60208 (e-mail: )
4. University of Florida, 1701 SW 16th Avenue, Gainesville, FL 32608 (e-mail: )
5. Massachusetts Institute of Technology, 50 Memorial Drive, Cambridge, MA 02142 (e-mail: )