1. Department of Economics, Harvard University, Littauer Center, 1805 Cambridge Street, Cambridge, MA 02138, and CEPR.
2. Centre for Economic Performance, London School of Economics, Houghton Street, London, WC2A 2AE, United Kingdom.
3. Chicago Booth Business School, University of Chicago, 5807 South Woodlawn Avenue, Chicago, IL 60637, and NBER.