1. Fuqua School of Business, Duke University, Durham, North Carolina 27708;
2. National Bureau of Economic Research, Cambridge, Massachusetts 02138;
3. Department of Management and Technology and the Invernizzi Center for Research on Innovation, Organization, Strategy and Entrepreneurship, Bocconi University, 20136 Milan, Italy;
4. Copenhagen Business School, DK-2000 Frederiksberg, Denmark;
5. Centre for Economic Policy Research, London EC1V 0DX, United Kingdom