Elder brothers affect the life history of younger siblings in preindustrial humans: social consequence or biological cost?

Sex-specific sibling interactions are potentially important in human ecology. It is well established that in patrilineal societies that sons suffer from the presence of brothers because of competition for inheritance. However, offspring (of both sexes) might also suffer from being born after an elder brother because of the greater costs sons may entail for their mother. Evidence that the cost of producing sons is higher has been gained from studies of ungulates and humans, with some of this cost being manifested as lower birthweight or reproductive performance of offspring born following a male. Using church record data from preindustrial Finland, we shed light on this process by investigating the demographic ‘mechanisms' by which offspring born following an elder brother are compromised. First, we show that, for both men and women in this population, being born after an elder male sibling is associated with reduced probability of reproducing, a later age at first reproduction, and longer interbirth intervals. Second, we show that the primary effect of interest is a reduced probability of reproducing in those born after an elder brother (even among only those who married). Finally, we show that the total number of elder brothers who survived to adulthood has a negative effect on male offspring only, and this effect is independent of the elder brother effect above. We highlight that differences in the success of human offspring are not always social in origin as is often perceived but can also be biological, resulting from differential costs for mothers of producing male versus female offspring.