IS SIBLING RIVALRY FATAL? Rebecca Kippen and Sarah Walters Is Sibling Rivalry Fatal? Siblings and Mortality Clustering The complex interrelationships between birth or- der, birth intervals, and sibling mortality clustering have been the source of much debate in historical demography and in the de- mography of the developing world. No clear resolution yet exists either about the relative importance of these factors in determin- ing infant and child mortality or about the pathways through which they operate. This article re-examines the issue in the context of nineteenth-century Belgium through the use of pop- ulation registers, which can go one step further than historical- demographic studies based on family-reconstitution data and con- temporary studies based on birth-history data. The movements in and out of households that population registers record enable us to trace the presence or absence of siblings in a household at a partic- ular point in time. Accommodating the number of siblings present in a household as a time-varying entity is important given that familial-mortality clustering may signal shared exposure to patho- gens and competition for resources, both of which depend on the presence of co-resident siblings. This article therefore aims to un- derstand child-mortality clustering though the medium of the Rebecca Kippen is Senior Research Fellow, Centre for Health and Society, University of Melbourne. She is the author of, with Peter A. Gunn, “Convict Bastards, Common-Law Unions and Shotgun Weddings: Premarital Conceptions and Exnuptial Births in Nineteenth- Century Tasmania,” Journal of Family History, XXXVI (2011), 387–403; with R. Alan Covey and Geoff Childs, “Dynamics of Indigenous Demographic Fluctuations: Lessons from Six- teenth-Century Cusco, Peru,” Current Anthropology, LII (2011), 335–360. Sarah Walters is Research Fellow in Demography, Population Studies Department, Lon- don School of Hygiene and Tropical Medicine. She is the author of, with Manuela Quaresma et al., “Geographical Variation in Cancer Survival in England, 1991–2006: An Analysis by Cancer Network,” Journal of Epidemiology & Community Health, LXV (2011), 1044–1052; with Bernard Rachet et al., “Population-Based Cancer Survival Trends in England and Wales up to 2007: An Assessment of the NHS Cancer Plan For England,” The Lancet Oncology, X (2009), 351–369. The authors thank several anonymous reviewers and the instructors and fellow partici- pants at the Longitudinal Methods in Historical Demography Workshop, University of Mich- igan, Ann Arbor (2007)—hosted by the International Consortium for Political and Social Re- search—for their helpful suggestions and comments. The authors also thank George C. Alter for permission to use the data on which this paper is based. © 2012 by the Massachusetts Institute of Technology and The Journal of Interdisciplinary History, Inc. Journal of Interdisciplinary History, xlii:4 (Spring, 2012), 571–591.