---- Acknowledgements ----
We thank the Southern California Innovation Project (Kauffman Foundation) for financial support. We thank Max Auffhammer for outstanding research assistance, and we appreciate helpful comments provided by participants in seminars at Russell Sage Foundation and the following universities: Carnegie-Mellon, Pennsylvania State, Pittsburgh, Syracuse, University of Southern California, and Yale. We are responsible for any errors. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research.