NBER Working Paper No. 18307
---- Acknowledgements ----
A version of this paper was presented as a plenary talk at the SED meeting in Gent, July 2011. I thank Michael Clemens, Eric French, two anonymous referees and many seminar participants for helpful comments. Xiaodong Fan provided valuable research assistance. The views expressed herein are those of the author and do not necessarily reflect the views of the National Bureau of Economic Research.