---- Acknowledgments ----
We are grateful to Ben Jones and Paula Stephan for their very helpful directions and to seminar participants for their comments. We also thank Alexis Brownell for excellent research assistance. Finally, we gratefully acknowledge financial support from the National Science Foundation, the Alfred P. Sloan Foundation, the Ewing Marion Kauffman Foundation, and Harvard Business School. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research.