1. Department of Economics, University College London, Gower Street, London WC1E 6BT, UK (e-mail: )
2. Department of Economics, University of California–Berkeley, 530 Evans Hall 3880, Berkeley, CA 94720-3880 (e-mail: )
3. Department of Economics and VCEE, University of Vienna, Oskar-Morgenstern-Platz 1, 1090 Vienna, Austria, and Department of Economics, CentER, and TILEC, Tilburg University (e-mail: )
4. Department of Economics, W. P. Carey School of Business, Arizona State University, PO Box 879801, Tempe, AZ 85287-9801, and NBER (e-mail: )