New research says that menopause evolved as a way to stop competition between a mother-in-law and her daughter-in-law.

A woman reaches menopause when she is 45 to 55 years old. However, why this happens is still unclear. A new study based on previous resarch has suggested that it might have to do with the mother-in-law and daughter-in-law relationship.

"Evolutionary theory expects animals to reproduce throughout their lifespan, and this is exactly what happens in almost every animal known, including human men. So why are women so different? Our study shows for the first time that the answer could lie in the relationship between a mother-in-law and daughter-in-law," said Dr. Andy Russell from the University of Exeter's Centre for Ecology and Conservation, co-author of the study.

The research was based on the 200 years of data about births and deaths that occurred in pre-industrialized Finland. The birth and death rates were obtained from church registries in Finland between 1700 and 1900.

The study found that a child born in a family where both mother-in-law and daughter-in-law were reproducing at the same time was twice as likely to die before the age of 15. This, however, did not hold true for mothers and daughters who were reproducing at the same time.

Researchers say that reproducing at the same time makes women competitive especially if they are not related.

The study also found that in families where a woman stopped reproducing after the age of 50, the woman had a higher chance of having more grandchildren. This could be explained by the fact that a non-reproducing mother-in-law will provide support and care for the daughter-in-law and her children. However, when the grandmother had babies, the chance of her baby and the daughter-in-law's baby surviving into adulthood went down by 50 percent.

"The research adds weight to the argument that menopause evolved because of the vital role that grandmothers played in rearing grandchildren in traditional societies. 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," said Dr. Virpi Lummaa, from the University of Sheffield's Department of Animal and Plant Sciences

Lumma added that in the modern society, fertile women avoid having babies when they become mother-in-laws.

The study was published in the journal Ecology Letters.