1. Department of Economics, University of Arizona, PO Box 210108, Tucson, AZ 85721, HEC Montreal, and NBER (e-mail: )
2. Department of Economics, Northwestern University, 2001 Sheridan Road, Evanston, IL 60208-2600, and NBER (e-mail: )
3. Wharton School, University of Pennsylvania, 3641 Locust Walk, Philadelphia, PA 19104, and NBER (e-mail: ).