1. Department of Economics, University of Minnesota, Twin Cities, 1035 Heller Hall, 271 19th Ave. South, Minneapolis, MN 55455, and National Bureau of Economic Research.
2. Department of Economics, University of Chicago, 1126 E. 59th St., Chicago, IL 60637.
3. Massachusetts Institute of Technology, 50 Memorial Drive, E52-262C, Cambridge, MA 02142, and NBER.