1. School of Public Health, Harvard University, 665 Huntington Avenue, Building 1, Room 1209, Boston, MA 02115, and Brookings Institution (e-mail: )
2. Department of Economics, Stanford University, 579 Serra Mall, Office 238, Stanford, CA 94305, and NBER (e-mail: )
3. Department of Economics, Dartmouth College, 328 Rockefeller Hall, Hanover, NH (e-mail: ).