1. Department of Economics, Massachusetts Institute of Technology, E52-380b, Cambridge, MA 02139, National Bureau of Economic Research, Centre for Economic Policy Research, and Canadian Institute for Advanced Research.
2. Department of Economics, Harvard University, Littauer 319, Cambridge, MA 02138, NBER, and CEPR.
3. Department of Economics, Harvard University, Littauer 229, Cambridge, MA 02138, NBER, CEPR, and CIAR.