1. Department of Economics, Boston University, 270 Bay State Road, Boston, MA 02215, USA
2. National Bureau of Economic Research, 1050 Massachusetts Avenue, Cambridge, MA 02138, USA
3. Department of Sociology, University of California, Berkeley, 496 Barrows Hall, Berkeley, CA 94720, USA
4. Department of Sociology, University of Minnesota, Twin Cities, 909 Social Sciences Building, 267 19th Avenue South, Minneapolis, MN 55455, USA