1. Harvard Kennedy School, Mailbox 114, 79 JFK Street, Cambridge, MA 02138, and NBER (e-mail: )
2. Department of Economics, MIT, 77 Massachusetts Avenue, Building E52, Room 442, Cambridge, MA 02139, and NBER (e-mail: )
3. Columbia University Mailman School of Public Health, 722 West 168 Street, 4th Floor, New York, NY 10032 (e-mail: )
4. Chicago Booth, 5807 South Woodlawn Avenue, Office 519, Chicago, IL 60637, and NBER (e-mail: )