Opposite effects of maternal and paternal grandmothers on infant survival in historical Krummhörn

On the basis of church register entries from the Krummhörn region (Ostfriesland, Germany, 1720-1874) we looked at the question whether the existence or non-existence of grandmothers had an impact on the reproductive success of a family. We found that fertility (measured by intervals between births) was not influenced by grandmothers. However, maternal grandmothers tended to reduce infant mortality when the children were between six and twelve months of age. During these six months, the relative risk of dying was approximately 1.8 times higher if the maternal grandmother was dead at the time of the child’s birth compared to if she was alive. Interestingly, the existence of paternal grandmothers approximately doubled the relative risk of infant mortality during the first month of life. We interpret this as being the result of a tense relationship between mother- and daughter- in-laws. We found that Krummhörn grandmothers could be both helpful and a hindrance at the same time. Geographic proximity tended to increase the effects found. If this ambivalent impact of grandmothers on familial reproduction could be generalized beyond the Krummhörn population, the hypothesis that the evolution of the postgenerative life span could be explained by grandmotherly kin-effects would have to be stated more precisely: the costs of social stress in the male descendency would have to be subtracted from the benefits of aid and assistance in the female descendency. At any rate, the Krummhörn data do not offer a role model for grandmothers who provide unconditional assistance, an effort which in itself could have explained the evolutionary extension of the human life span.