1. Toulouse School of Economics, 21 allée de Brienne, 31015 Toulouse Cedex 6 France (e-mail: )
2. Institute for Fiscal Studies and the University of Manchester, 7 Ridgmount Street, London WC1E 7AE UK (e-mail: )
3. Department of Economics, Northwestern University, 2001 Sheridan Road, Evanston, IL 60208-2600, and NBER (e-mail: )