1. Department of Economics, Harvard University, Littauer Center, 1805 Cambridge Street, Cambridge, MA 02138, and National Bureau of Economic Research.
2. Harvard Kennedy School, Taubman Center 356, 79 John F. Kennedy Street, Cambridge, MA 02138, and National Bureau of Economic Research.
3. University of California, 530 Evans Hall #3880, Berkeley, CA 94720, and National Bureau of Economic Research.