1. Department of Economics, University of Texas at Austin, TX 78712, Norwegian School of Economics, Institute for the Study of Labor (IZA) and National Bureau of Economic Research (NBER) (e-mail: )
2. School of Economics and Geary Institute, University College Dublin, Newman Building, Belfield, Dublin, Ireland, The Center for Economic Policy and Research (CEPR), and IZA (e-mail: )
3. Department of Economics, Norwegian School of Economics, Helleveien 30, N-5045 Bergen, CEPR, Center for the Economics of Education (CEP), CES-IFO and IZA (e-mail: )