The menopause partly evolved to prevent competition between mothers and their daughter-in-laws, according to researchers.
A study found that when a grandmother had a baby later in life, and at the same time as her daughter-in-law, both newborns were 50 per cent less likely to survive to adulthood.
The researchers, from the University of Sheffield and Turku University in Finland among others, said it could explain why women stop reproducing so early in life unlike most other animals.
It also adds weight to the theory that the menopause evolved to allow women to focus on their grandchildren. Traditionally, this role included providing food for the family and protecting young children from accidents and disease.
The topic has rarely been analysed, because it requires detailed data on the reproductive success of several generations of women, with knowledge on who lived with whom and when.
Scientists analysed 200-years' worth of data collected by Dr Virpi Lummaa of the University of Sheffield and her student Mirkka Lahdenperä of Turku University, Finland, from church registers of pre-industrial Finland.
They looked at information on birth and death rates from 1700 to 1900, before the advent of modern contraception or healthcare.
The study reveals that women had more grandchildren if they stopped reproducing around the age of 50. The research team believes this was partly because of reduced competition between the older woman and her daughter-in-law and partly because of the support she could offer her grandchildren.
A child born to families with a mother-in-law and daughter-in-law reproducing simultaneously was twice as likely to die before reaching the age of 15. However, this was not the case in the instances when a mother and daughter had babies at the same time. This suggests that related women breed cooperatively and unrelated women breed conflictually.
There is a clear biological benefit to a woman cooperating with her daughter: the women share 50 per cent of the same genes so being in competition for food and other resources makes little sense.
This is not the case for a mother-in-law and daughter-in-law: they are not related, so it is logical they should compete to maximise on their chances of spreading their genes.
Consequently, the Finnish data shows that the average woman would benefit from stopping reproducing at the age of 51 if she risked breeding with her daughter-in-law, but not her daughter.
Different theories have been put forward for the evolution of the menopause in humans, including the idea that it evolved to protect older women against the danger of dying during pregnancy or childbirth.
However, under two per cent of the pre-industrial Finns in this study died in childbirth in their mid-40s, and such risks of dying in childbirth are similarly low in hunter-gatherers today.
Dr Virpi Lummaa, from the University of Sheffield said: 'Although family roles have changed, many grandmothers still play a vital role in caring for their grandchildren and in western society a large number provide daycare.
'It is interesting that even today, mothers rarely choose to have children at the same time as their offspring: even if they have not yet been through the menopause.'