The Return to Knowledge Hierarchies
---- Acknowledgements -----
We thank Matthew Gentzkow and participants at various seminars, the 2006 Summer Meetings of the Econometric Society, and NBER Summer Institute for comments. We are especially grateful to Steve Tadelis, who presented this paper and offered extensive comments at the latter. The research in this paper was conducted while the authors were Census Bureau research associates at the Chicago Research Data Center. Research results and conclusions are those of the authors and do not necessarily indicate concurrence by the Bureau of the Census. This paper has been screened to ensure that no confidential data are revealed. The views expressed herein are those of the author(s) and do not necessarily reflect the views of the National Bureau of Economic Research.