1. University of Chicago, 1307 East 60th Street, Chicago, IL 60637, and NBER (email: )
2. Johns Hopkins University SAIS, 1717 Massachusetts Ave NW, Washington, DC 20036 (email: )
3. Economics Department, Dartmouth College, 6106 Rockefeller Center, Room 301, Hanover, NH 03755 (email: )