2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Scientists have once again confirmed the "grandmother hypothesis", according to two articles (1, 2) published in Current Biology. In 17th-19th century Finnish families, having a maternal grandmother was associated with a 30 percent increase in the survival rate of their grandchildren under the age of five. But the presence of grandmothers on the paternal side did not affect her in any way. In 17th-18th century Canadian families, the presence of maternal grandmothers was associated with more children. However, the “grandmother effect” disappeared if relatives lived far from each other.
Women live long after menopause. This can be attributed to advances in modern medicine, but perhaps the longer life span was due to evolutionary advantages. According to the “grandmother hypothesis,” children cared for by older women living in the family survived more often. The researchers found that the survival rate of grandchildren depended on the age of the child, and on whether the grandmother took care of them, from the father's side or from the mother's side. However, until now, scientists have not analyzed whether a woman's age influences the “grandmother effect”. It was also unclear if it persists if the grandmother lives far from her grandchildren.
To analyze whether the “grandmother effect” depends on her age, biologists at the University of Turku, led by Virpi Lummaa, examined the records in Finnish church books from 1731-1895. They looked at child survival and analyzed whether it depended on the presence of grandmothers and their age. In total, the researchers analyzed 5815 grandchildren.
The Finns who lived at that time had large families, with an average of 5, 5 children per family. But a third of the children died before they reached the age of five, and almost half - before they reached the age of 15. Mostly they died from infectious diseases, including measles and smallpox. The average life expectancy for adults was over 60 years. At the same time, more than half of women who had at least one child lived to be 50 years old and managed to raise grandchildren up to 5-10 years of age. Most of the country's inhabitants were engaged in agriculture. Usually the eldest son inherited the family farm, while his brothers and sisters lived in the same area. So both grandmothers often lived nearby, and the husband's mother usually shared the same house with his family.
It turned out that maternal care of 50-75-year-old grandmothers was associated with a lower mortality rate for their grandchildren. In such families, children 2–5 years old were 29.5 percent more likely to survive. The presence of grandmothers in the family on the part of the father was not associated with the health of the grandchildren. And the husband's mother over 75, who lived in the family, on the contrary, was associated with a worsening of the situation. In such families, the survival rate of children under two years old fell by 37.1 percent.
At the same time, the researchers analyzed the life expectancy of the women themselves. It began to decrease when those were over 60, and most of them have already had all their grandchildren. At 70, the risk of death increased by three, and at 80, it was six times higher than at 50. Presumably, it was the level of modern medicine that made it possible to significantly increase life expectancy (female residents of modern Finland have a life expectancy of 84.2 years).
In a second study, Canadian biologists and anthropologists led by Patrick Bergeron of the University of Bishops analyzed the Registre de la Population du Québec Ancien, which contained information on French settlers from 1608 to 1799. The researchers analyzed records from 149 parishes in the St. Lawrence Valley of 3,382 grandmothers who had 7,164 married daughters and 56,767 grandchildren.
Scientists have also noticed the "grandmother effect". Those women whose mothers were alive when they gave birth to their first child gave birth to their first child four months earlier, and, on average, gave birth to two more children than women whose mothers died. The presence of a maternal grandmother in the family increased the survival rate of children up to 15 years by an average of 1, 14 children (in families of French settlers there were, on average, 10, 2 children). The researchers suggested that the further the mother and daughter lived, the weaker the “grandmother effect” was. And so it turned out. Families that lived 325 kilometers from their maternal grandmothers had, on average, 1.75 fewer children, and the survival rate of children was lower (on average, 1.45 children) compared to families who lived in next door to grandmothers. If the relatives lived closer, the picture changed. In families that lived 100 and 25 kilometers from grandmothers, on average, there were 0, 58 and 0, 14 children less, and survived by 0, 48 and 0, 12 fewer children, respectively, than in families who lived next to grandmothers …
Earlier anthropologists linked the emergence of the institution of marriage to the influence of the “grandmothers effect”. As shown by the mathematical model that the researchers built, the increase in the life expectancy of women led to its increase in the entire population as a whole. Competition for young, fertile partners became higher and they needed to be protected from competitors. As a result, this led to the emergence of a long-term pair relationship.