1. Research Centre for Education and the Labour Market, Maastricht University, Tongersestraat 53, 6211 LM Maastricht, Netherlands.
2. Department of Economics, University of Bonn, Adenauerallee 24-42, 53113 Bonn, Germany.
3. Department of Economics, Swarthmore College, 500 College Avenue, Swarthmore, PA 19081-1397.
4. Department of Economics, University of St. Gallen, Varnbuelstr. 14, 9000 St. Gallen, Switzerland.