1. University of Paris, Dauphine, Placo de Marechal de Lattre de Tassigny, 75775 Paris Cedex 16, France.
2. Department of Economics, University of Chicago, 1126 E. 59th Street, Chicago, IL 60637.
3. Institute for Fiscal Studies and the Department of Economics, University College London, Gower Street, London, WC1 E6BT, United Kingdom.