Spa, M.; Young, E. A.; Lummaa, V.; Postma, E.; Dugdale, H.

Siblings are an important part of an individual\'s early-life environment and may therefore play an important role in shaping an individual\'s fitness. The quantification of sibling effects is challenging, especially in long-lived species with extended parental care and overlapping generations, such as humans. Here we quantify how the survival status, age, and sex of older siblings shape childhood survival across 2941 focal individuals born between 1750-1870 using historical parish data from Switzerland. While the total number of older siblings was not associated with an individual\'s childhood survival, distinguishing between siblings by their survival status, age, and sex revealed several associations, which in some cases also interacted with the sex of the focal individual: While older brothers close in age reduced the survival of girls but not boys, having more older sisters born close in age improved their younger sibling\'s survival. Our results, therefore, show that sibling interactions play an important role in shaping early-life survival, and highlight that the strength and direction of these effects are context-dependent and can arise through biological and cultural factors. We encourage future studies on sibling interaction to consider siblings\' survival, age and sex, in both humans and other species.