1. Department of Economics, Duke University, Durham, North Carolina 27708-0097;
2. National Bureau of Economic Research, Cambridge, Massachusetts 02138
3. Department of Economics, Kellogg School of Management, Northwestern University, Evanston, Illinois 60208-2600;,
4. Federal Reserve Bank of Chicago, Chicago, Illinois 60604
5. Center for Economic Policy Research, London, EC1V 3PZ, United Kingdom.