1. Department of Economics, Massachusetts Institute of Technology, E52-380B, 50 Memorial Drive, Cambridge, MA 02142, and Canadian Institute for Advance Research (CIFAR).
2. Department of Economics and Quantitative Methods, University of Urbino, Via Saffi 42, 61029, Urbino, Italy.
3. Department of Politics, Princeton University, 037 Corwin Hall, Princeton University, Princeton, NJ 08544, and the Institute for the Study of Labor.