The Grandmother Effect Suggests That Proximity Is a Factor in Family Size

By Bergeron, Patrick; professor, Associate et al. | The Canadian Press, May 10, 2019 | Go to article overview

The Grandmother Effect Suggests That Proximity Is a Factor in Family Size


Bergeron, Patrick, professor, Associate, University, Bishop's, The Canadian Press


The grandmother effect suggests that proximity is a factor in family size

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This article was originally published on The Conversation, an independent and nonprofit source of news, analysis and commentary from academic experts. Disclosure information is available on the original site.

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Author: Patrick Bergeron, Associate professor, Bishop's University

The human species is one of the very few, along with orcas and pilot whales, where females cease to be fertile after approximately 45 years of age. Since the ultimate goal of any living organism is to spread their genes, the evolution of menopause in women has been quite puzzling for scientists.

But what if menopause actually favoured collaboration between generations? The grandmother effect hypothesis suggests that menopausal women might still be able to increase their genetic footprint, despite not reproducing anymore. They do this by helping their children to raise larger families.

Let's do the math -- parent and offspring share 50 per cent of their genes, grand-parents and grandchildren 25 per cent. It means that an increase by only two grandchildren would be about the same as producing an offspring of your own.

Historical human population data are valuable to study the grandmother hypothesis because detailed life-history events are recorded over multiple generations and family relationships are well documented. The positive impact of the presence of a living grandmother on her daughter's family size has recently been shown in a study of pre-industrial populations of Quebec, shedding light on mechanisms that could explain the evolution of menopause.

Tough life in Nouvelle France

My research team, which includes first author post-doctoral fellow Sacha Engelhardt from the Universite de Sherbrooke, used the population register of French-Canadian ancestors from 1608 to 1799 carefully assembled by demographers. The Catholic Church kept meticulous records of births, marriages and deaths, painting a unique portrait of this fairly isolated population.

Families were very large, with an average of eight children per family and some women bore up to 20 children. Of these, many would die before their first birthday. In some years, infant deaths rose above 30 per cent.

It is interesting to ponder the factors that might counter such a frightening statistic, especially by looking at the beneficial role that family members -- in this case grandmothers -- can have on their relatives. The challenge was not only to have many kids, but also to ensure that they could reach sexual maturity and become parents themselves. Some help could definitely make a difference.

A grandmother alive leads to more grandchildren

Having a living maternal grandmother increased the number of offspring born by their daughters by about 20 per cent. …

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