1. Department of Economics, Stanford University, 578 Serra Mall, Palo Alto, CA 94350 and CEP and NBER.
2. Graduate School of Business, Harvard University, Soldiers Field, Boston, MA 02163 and CEP.
3. Department of Economics, London School of Economics, Houghton Street, London, WC2A 2AE and CEP, CEPR, and NBER.