---- Acknowledgements -----
We are grateful to Pierre-Andre Chiappori, Edward Glaeser, Lawrence Katz, Bernard Salanie and two anonymous referees for helpful comments and suggestions. We also thank seminar participants at numerous universities. Loury gratefully acknowledges financial support from the Andrew W. Mellon Foundation and the Carnegie Corporation. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research.