Do post-reproductive aged females promote maternal health? Preliminary evidence from historical populations

Abstract

Background and objectives: Much literature argues that natural selection conserved menopause and longevity in women because those who stopped childbearing helped bolster daughters' fertility and reduce infant mortality among grandchildren. Whether the presence of grandmothers ever improved fitness sufficiently to affect longevity via natural selection remains controversial and difficult to test. The argument underlying the grandmother and associated alloparenting literature, however, leads us to the novel and testable prediction that the presence of older women in historical societies could have affected population health by reducing lethality associated with childbearing.

Methodology: Using historical life table data from four societies (Denmark, England and Wales, France and Sweden), we test the hypothesis that death rates among women initiating childbearing declined when the societies in which they were embedded included unexpectedly high frequencies of older women. We use time series analysis to measure the extent to which the observed likelihood of death among women aged 20-24 differs from statistically expected values when the number of older women grows or declines.

Results: In three of the four countries examined, we find an inverse relationship between the frequency of post-reproductive females in the population and odds of mortality among females at the peak of childbearing initiation.

Conclusions and implications: Results suggest that the presence of older women in a population may enhance population health by reducing mortality among women who face high risk of maternal death, although additional research is needed to determine if this relationship is causal.

Keywords: alloparenting; collective breeding; grandmothers; historical demography; intergenerational transfers; maternal health.

Figure 1.

First differences of the number of women (in 1000 s) of women aged 50–69 for Denmark (1835–1913), England and Wales (1841–1913), France (1816–1913) and Sweden (1751–1913)

Figure 2.

Observed and expected natural log of the death rate among 20–24-year-old women for Denmark (1835–1913), England and Wales (1841–1913), France (1816–1913) and Sweden (1751–1913)