Abstract
Grandmothers provide key care to their grandchildren in both contemporary and historic human populations. The length of the grandmother-grandchild relationship provides a basis for such interactions, but its variation and determinants have rarely been studied in different contexts, despite changes in age-specific mortality and fertility rates likely having affected grandmotherhood patterns across the demographic transition. Understanding how often and long grandmothers have been available for their grandchildren in different conditions may help explain the large differences between grandmaternal effects found in different societies, and is vital for developing theories concerning the evolution of menopause, post-reproductive longevity, and family living. Using an extensive genealogical dataset from Finland spanning the demographic transition, we quantify the length of grandmotherhood and its determinants from 1790–1959. We found that shared time between grandmothers and grandchildren was consistently low before the demographic transition, only increasing greatly during the 20th century. Whilst reduced childhood mortality and increasing adult longevity had a role in this change, grandmaternal age at birth remained consistent across the study period. Our findings further understanding of the temporal context of grandmother-grandchild relationships, and emphasise the need to consider the demography of grandmotherhood in a number of disciplines, including biology (e.g. evolution of the family), sociology (e.g. changing family structures), population health (e.g. changing age structures), and economics (e.g. workforce retention).
Highlights
In cooperatively-breeding species across a range of taxa, offspring are cared for by the parents, and by other available group members. Who helps in such cooperative societies is determined by a range of factors affecting helper willingness to help, including relatedness [1] and ecological conditions [2], as well as those that may affect the availability of different alloparents to provide care, such as sex differences in dispersal or lifespan
Grandmother physical and mental health, for example, can be affected by the level of care they provide to grandchildren [21,22]; grandmother happiness in Finland positively correlates with increasing contact [23], whilst in the US, those co-residing with a grandchild are more likely to suffer from depressive symptoms [24]
We first assessed the percentage of individuals with a grandmother alive at birth and through their childhood, and found evidence that more than a third of grandchildren across the entire study period in our sample were not able to interact at all with their grandmother: 36.2% of children were born without a living maternal grandmother, and 43.6% without a living paternal grandmother
Summary
In cooperatively-breeding species across a range of taxa, offspring are cared for by the parents, and by other available group members (i.e. alloparents). Caregiving grandparents in contemporary Germany had a decreased mortality risk compared to non-caregiving grandparents and to non-grandparents [25], albeit grandmothers were not distinguished from grandfathers in this study. Whether such health effects are a universal condition of grandmothering in humans and are common across different conditions is unknown at present, as most studies on the health impacts of grandmothering are confined to affluent industrialised countries
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