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How mothers-in-law got a bad name

September 11, 2002

If relations with the in-laws are a little strained, spare a thought for German peasants in the 18th and 19th centuries. For them, having Dad`s mother around could double the chances of a child dying.

From an evolutionary point of view, it makes sense for a mother`s parents to take more of an interest in a grandchild than the father`s parents. While mum`s mother can be sure she`s cooing at her own flesh and blood, there is always the chance that dad- and his parents- have been duped into looking after the milkman`s kid.

Studies often find that paternal grandmothers have less of a positive influence on a child`s health, but for the first time, they have now been shown to have a negative effect.

Eckart Voland at Giessen University in Germany and his colleague Jan Beise leafed through church registers from the Krummhšrn region of northern Germany for birth and death data on low-income families. They found that if a mother`s 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`s development because the mother`s mother helps her with weaning. But if the father`s mother was around, the child was half as likely to survive as they would have been if she were dead.

Voland thinks that in the strict religious society then prevalent in the region, a man`s mother might have been overly suspicious of a baby`s paternity. The harassment of the mother may have had a negative effect on the child`s care. So the "evil mother-in-law", 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. "The son can also get grandchildren by having sexual relations with other women," he says. This would be in the grandmother`s interests, provided the mother continues to care for the child. It might pay the father`s grandmother to destabilise the relationship, freeing her son to explore pastures new.

New Scientist