NBER Working Paper No. 18499
---- Acknowledgements -----
We gratefully acknowledge the financial support of the National Science Foundation through its SciSIP Program (Awards SBE-0738142 and SBE-0738394). We thank Heidi Williams, Peter Thompson, and various seminar audiences for insightful comments. Lisa Bassett, Vivienne Groves, James Sappenfield and Mikka Rokkanen provided additional research assistance. The project would not have been possible without Andrew Stellman's extraordinary programming skills (www.stellman-greene.com). All errors are our own. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research.