Humans, pilot whales and killer whales are the only animals known to stop being able to reproduce long before they die. In terms of evolution, where passing on your genes is the main reason for living, the menopause remains puzzling.

Now, using a large data set from Finland, researchers have for the first time been able to test a hypothesis that competition between different generations of genetically unrelated breeding women could have promoted the evolution of the menopause. The results are published today in Ecology Letters1.

Mirkka Lahdenperä, an ecologist at the University of Turku in Finland, and her colleagues used data from meticulous birth, death and marriage records kept by the Lutheran church in the country between 1702 and 1908. As they dug into the data, the researchers found that the chances of children dying increased when mothers-in-law and daughters-in-law gave birth around the same time. For children of the older women, survival dropped by 50%. For children of the daughters-in-law, it dropped by 66%. However, if mothers and daughters had children at the same time, the survival of those children wasn’t affected.

The results suggest that it would be beneficial to stop having children once your daughter-in-law entered the fray. “We were surprised that the result was so strong,” says Andrew Russell, an ecologist at the University of Exeter, UK, who was part of the research team. He suggests that perhaps in-laws fought over food for their children instead of cooperating as mothers and daughters might.