1. Department of Demography, University of California, Berkeley, 2232 Piedmont Avenue, Berkeley, CA 94720-2120, USA
2. Department of Sociology, University of Washington, Seattle, 211 Savery Hall, Box 353340, Seattle, WA 98195-3340, USA
3. Institut National d’Études Démographiques, 133 Boulevard Dabout, 75020 Paris Cedex, France