1. University of Chicago, Chicago, IL 60637 (e-mail: )
2. Harvard University, Cambridge, MA 02138 (e-mail: )
3. New York University, New York, NY 10003 (e-mail: )
4. Harvard Business School, Boston, MA 02163 (e-mail: )
5. University of Chicago, Chicago, IL 60637, and NBER (e-mail: )
6. Harvard University, Cambridge, MA 02138, and NBER (e-mail: )