1. Department of Economics, Stanford University, 579 Serra Mall, Stanford, CA 94305-6072, Centre for Economic Performance, CEPR, and NBER.
2. Harvard Business School, Soldiers Field, Boston, MA 02163, Centre for Economic Performance and NBER.
3. London School of Economics, Centre for Economic Performance, Houghton Street, London WC2A 2AE, UK, CEPR, and NBER.