1. Department of Community Health, Tufts University, Medford, MA 02155
2. Max Planck Institute for Demographic Research, Rostock 18057, Germany
3. Population Research Unit, University of Helsinki, Helsinki 00014, Finland
4. Max Planck–University of Helsinki Center for Social Inequalities in Population Health, Rostock 18057, Germany
5. Department of Epidemiology, University of Texas Medical Branch, Galveston, TX 77555