MOTHERS-IN-LAW are the butt of many jokes. There may, though, be a sinister reality behind the humour. A report in this month's Behavioural Ecology and Sociobiology, by Jan Beise of the Max Planck Institute for Demographic Research in Rostock and Eckart Voland of Giessen University, both in Germany, suggests that, in the past at least, a mother-in-law may have had a fatal effect on her daughter-in-law's children.

Dr Beise and Dr Voland drew their data from church registers in the Krummhorn region of Germany. They studied the records of more than 23,000 families who lived there between 1720, when the records became comprehensive, and 1874, when the job of keeping them was taken away from the church. They found that the chance of a child dying within a month of birth doubled if the child's paternal grandmother was alive. The effect did not depend on whether the grandmother was living in the same household as the child.

Maternal grandmothers, by contrast, made no difference during the first month, and made a significant improvement to a child's prospects later on. The odds of a child who reached the age of six months going on to celebrate a first birthday were 60% higher if the child's maternal grandmother was still alive.

This second result makes perfect evolutionary sense. The first, though, does not. After all, a paternal grandmother has as much genetic interest in the success of her son's children as a maternal one does in her daughter's. But only, of course, if they really are her son's children. The maternal grandmother is the only grandparent who can be sure she is really related to her grandchildren. The paternal grandmother shares, in an evolutionary sense, her son's risk that he might have been cuckolded. So the best explanation that the two researchers can come up with is that women are in some way harassed by their mothers-in-law to discourage adulterous matings.

Such harassment might well have an adverse effect on a woman's fetus if she happened to be pregnant, and that effect would probably show up early in a child's life. And, witness the jokes, harassment is widely perceived to take place. But there is not much evolutionary point in “guarding” a pregnant woman against such matings, since she cannot become pregnant again. So the mother-in-law effect remains a bit of a mystery.