About the Authors
Department of Society, Human Development and Health, Harvard School of Public Health, Boston, Massachusetts, United States of America
Department of Statistics, Harvard School of Public Health, Boston, Massachusetts, United States of America
Division of Research, Kaiser Permanente, Oakland, California, United States of America
The authors have declared that no competing interests exist.
NK conceived and designed the study, interpreted the data, and prepared the manuscript. JTC helped devise the analytic plan, conducted the analyses, and assisted with interpreting the data and preparing the manuscript. BAC advised on the analytic plan, and assisted with analyzing and interpreting the data and with preparing the manuscript. JVS designed the Kaiser Permanente Women Twins Study II, and assisted with interpretation of the data and preparing the manuscript.