1. University of Illinois at Urbana-Champaign, 340 Wohlers Hall, MC-706, 1206 S. Sixth Street, Champaign, IL 61820, and NBER.
2. Brookings Institution, 1775 Massachusetts Avenue, NW, Washington, DC 20036, and NBER.
3. Department of Economics, Harvard University, Littauer 208, Cambridge, MA 02138, and NBER.
4. Institute for Quantitative Social Science, Harvard University, 1737 Cambridge Street, Cambridge, MA 02138.