---- Acknowledgments ----
We thank Andy Atkeson, David Atkin, Lorenzo Caliendo, Arnaud Costinot, Jonathan Dingel, Dave Donaldson, Pablo Fajgelbaum, John Geanakoplos, Penny Goldberg, Sam Kortum, Xiangliang Li, Giovanni Maggi, Kiminori Matsuyama, Ralph Ossa, Steve Redding, Andres Rodriguez-Clare, Chris Tonetti, and numerous seminar and workshop participants for excellent comments and suggestions. A Matlab toolkit which is the companion to this paper is available on Allen's website. 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.