The research reported here was supported in part by the National Science Foundation's Science of Science and Innovation Policy Program under award 1064218. We have benefited from the insightful comments of Irwin Feller, David Popp, Otavio Bartalotti, participants in the Iowa State University "no free lunch" colloquium and several anonymous referees. We wish to express our appreciation for their assistance. Needless to say, any remaining errors are the responsibility of the authors. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research. NBER working papers are circulated for discussion and comment purposes. They have not been peer-reviewed or been subject to the review by the NBER Board of Directors that accompanies official NBER publications.