The regular presence of a paternal grandmother can double the chance of a child dying 鈥� at least, it did for German peasants in the 18th and 19th centuries.
From an evolutionary point of view, it makes sense for a mother鈥檚 parents to take more of an interest in a grandchild than the father鈥檚 parents. While mum鈥檚 mother can be sure she鈥檚 cooing at her own flesh and blood, there is always the chance that dad 鈥� and his parents 鈥� have been duped into looking after the milkman鈥檚 kid.
Studies often find that paternal grandmothers have less of a positive influence on a child鈥檚 health, but for the first time, they have now been shown to have a negative effect.
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Eckart Voland at Giessen University in Germany and his colleague Jan Beise leafed through church registers from the Krummhregion of northern Germany for birth and death data on low-income families.
They found that if a mother鈥檚 mother was alive when the child was between 6 and 12 months old, it was 79 per cent more likely to survive than if she were dead. Voland thinks the positive effect is strong at this point in the child鈥檚 development because the mother鈥檚 mother helps her with weaning. But if the father鈥檚 mother was around, the child was half as likely to survive as they would have been if she were dead.
Pastures new
Voland thinks that in the strict religious society then prevalent in the region, a man鈥檚 mother might have been overly suspicious of a baby鈥檚 paternity. The harassment of the mother may have had a negative effect on the child鈥檚 care. So the 鈥渆vil mother-in-law鈥�, while rooted in evolutionary conflict, is probably more a cultural phenomenon, he says.
But Harald Euler, an expert in the evolution of family relationships at Kassel University in Germany, thinks there may be more going on.
鈥淭he son can also get grandchildren by having sexual relations with other women,鈥� he says. This would be in the grandmother鈥檚 interests, provided the mother continues to care for the child. It might pay the father鈥檚 mother to destabilise the relationship, freeing her son to explore pastures new.