1. Population Studies Center, Institute of Social Research; and Department of Health Behavior and Health Education, School of Public Health, University of Michigan, 4426 Thompson Street, Ann Arbor, MI 48106-1248, USA
2. Department of Economics; and Population Studies Center, Institute for Social Research, University of Michigan, 426 Thompson Street, Ann Arbor, MI 48106-1248, USA
3. Department of Population Health and Disease Prevention, Program in Public Health, UC Irvine, Anteater Instruction and Research Building (AIRB), 653 E. Peltason Road, Irvine, CA 92697-3957, USA