1. Yale University, Department of Epidemiology and Public Health, New Haven, Connecticut
2. University of Pennsylvania, School of Dental Medicine and Leonard Davis Institute of Health Economics, Philadelphia, PA
3. Alfred I. duPont Hospital for Children, Wilmington, Delaware
4. Alfred 1. duPont Hospital for Children, Wilmington, Delaware; University of Pennsylvania, W. M. Krogman Center for Research in Child Growth and Development and Leonard Davis Institute of Health Economics, Philadelphia, PA