1. University of California, Berkeley, 2607 Hearst Ave., #7320, Berkeley, CA 94720, and NBER (e-mail: )
2. Northwestern University, 2120 Campus Drive, Evanston, IL 60208, and NBER (e-mail: )
3. Columbia University, 420 West 118th Street, New York, NY 10027 (e-mail: )