Virpi Lummaa’s group has another paper, Offspring fertility and grandchild survival enhanced by maternal grandmothers in a pre-industrial human society:

Help is directed towards kin in many cooperative species, but its nature and intensity can vary by context. Humans are one of few species in which grandmothers invest in grandchildren, and this may have served as an important driver of our unusual life history. But helping behaviour is hardly uniform, and insight into the importance of grandmothering in human evolution depends on understanding the contextual expression of helping benefits. Here, we use an eighteenth-nineteenth century pre-industrial genealogical dataset from Finland to investigate whether maternal or paternal grandmother presence (lineage relative to focal individuals) differentially affects two key fitness outcomes of descendants: fertility and survival. We found grandmother presence shortened spacing between births, particularly at younger mother ages and earlier birth orders. Maternal grandmother presence increased the likelihood of focal grandchild survival, regardless of whether grandmothers had grandchildren only through daughters, sons, or both. In contrast, paternal grandmother presence was not associated with descendants’ fertility or survival. We discuss these results in terms of current hypotheses for lineage differences in helping outcomes.

The basic finding is that in Finland maternal grandmothers increase the fitness of their grandchildren. This is a big finding in Lummaa’s work but in the discussion this paper notes in other cultures a paternal grandmother effect may be operative. So how general is this result? Do we believe in the maternal grandmother effect?

Also, despite the possibility of a maternal grandmother effect, societies in the last 5,000 years seem to have shifted to strong patrilocality and patrilineality. Is this a multi-level selection problem? Basically, inter-group competition between groups hash out so that patrilineality wins on that scale, but within the groups, maternal grandmothers are more important?

Virpi Lummaa’s group has another paper, Offspring fertility and grandchild survival enhanced by maternal grandmothers in a pre-industrial human society: Help is directed towards kin in many cooperative species, but
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Anthroplogy