1. Department of Economics, Massachusetts Institute of Technology, Building E52, Room 438, 77 Massachusetts Avenue, Cambridge, MA 02142, and NBER (email: )
2. Department of Economics, University of Zürich, Schoenberggasse 1, CH-8001, Zürich, Switzerland, and CEPR (email: )
3. University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0519, and NBER (email: )