1. Center for Experimental Social Science and Department of Economics, New York University, 19 W.4th Street, 6FL, New York, NY 10012, NBER, and Research Institute of Industrial Economics (IFN) (email: )
2. Department of Economics, Stockholm School of Economics, Box 6501, SE-113 83 Stockholm, Sweden, and IFN (email: )
3. Northwestern University, 2001 Sheridan Road, Evanston, IL 60208, and NBER (email: )
4. Institute for International Economic Studies, Stockholm University, SE-106 91 Stockholm, Sweden (email: )