OBJECTIVE--To examine whether contracting an infection from a sibling of the opposite sex affects child mortality. DESIGN--Retrospective study of twins and case-control study of randomly selected children conducted within a prospective cohort study. SETTING--Urban and rural areas of Guinea-Bissau. SUBJECTS--251 Pairs of twins (65 male pairs, 72 female pairs, and 114 mixed pairs), and 39 children who died and 78 controls identified in a cohort of 853 children aged less than 4, comprising 772 randomly selected children, all twins, and all children who had measles living in Bandim 2. MAIN OUTCOME MEASURES--Death of a child and sex of cotwin or older sibling nearest in age. RESULTS--The mortality among children from mixed pairs of twins was higher than that among pairs of the same sex (relative risk 1.47, 95% confidence interval 1.14 to 1.90). Boys had a slightly higher mortality especially in the older groups. In the case-control study the older sibling nearest in age was of the opposite sex for 29 (74%) of the children compared with 36 (46%) of the controls matched for age, sex, and area. Mortality was higher among both boys and girls with older siblings of the opposite sex than among those with siblings of the same sex (odds ratio 1.90, 0.78 to 4.61 v 7.5, 1.59 to 35.30), but the difference was more pronounced among girls. CONCLUSIONS--Children whose older sibling nearest in age is of the opposite sex or who have cotwins of the opposite sex have a higher mortality from infectious diseases. Further investigation is needed to determine the nature of this relation
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