[1] Univ Calif Los Angeles, Los Angeles, CA 90095 USA
[2] Natl Bur Econ Res, Cambridge, MA 02138 USA
[3] Yale Univ, New Haven, CT 06520 USA
[4] World Bank Grp, Washington, DC USA
[5] Univ Calif Berkeley, Berkeley, CA 94720 USA
[6] Columbia Univ, New York, NY 10027 USA