1. George Washington University, 2115 G Street, Suite 364, Washington, DC 20052 (e-mail: )
2. University of Zurich, Schönberggasse 1, 8001 Zürich, CEPR, and NBER (e-mail: )
3. Department of Economics, University of California, Davis, One Shields Avenue, Davis, CA 95616, and NBER (e-mail: )