1. Department of Economics, University College London, Gower Street, London WC1E 6BT, United Kingdom and National Bureau of Economic Research (NBER).
2. Georgetown Public Policy Institute, Georgetown University, Old North Suite 311, 37th and O Streets, N.W., Washington, DC 20057 and NBER.
3. Department of Economics, Yale University, Box 208264, New Haven, CT 06520-8264 and University College London.