1. Department of Economics, Stockholm School of Economics, P.O. Box 6501, Sveavägen 65, SE-113 83 Stockholm, and CEPR (email: )
2. Department of Economics, Trinity College Dublin, Arts Building, Dublin 2, Ireland, and LICOS (email: )
3. Institute for International Economic Studies, Stockholm University, 106 91 Stockholm, Sweden, and CEPR (email: )
4. Department of Economics, University of Zurich, Schönberggasse 1, 8001 Zurich, Switzerland (email: )