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This is Scientific American — 60-Second Science. I'm Karen Hopkin.

Living with your parents has its benefits...at least when it comes to raising your kids—their grandkids. Because two new studies add to the evidence that grandmothers can enhance the survival of grandchildren. That is, unless grandma's too old or lives too far away. The results appear in the journal Current Biology.

Humans are unusual in that the females live long past the age at which they stop having babies.

"We don't really see that in nature. Most of the organisms will reproduce up to their very last moment."

Patrick Bergeron, professor of biology at Bishop's University in Quebec.

This increase in post-reproductive longevity¹ is often explained by the so-called “grandmother effect.

"Because family members share their genes², there could still be a benefit for postmenopausal women to increase their genetic³ footprint by helping⁴ their daughters to rear larger families."

To explore the “grandmother effect” hypothesis, Bergeron and his colleagues examined nearly 200 years' worth of French-Canadian population records from the 17th and 18th centuries.

"At the time, life was tough. In some years, a third of the kids were not even making it to one year of age."

But the researchers found that having a grandmother still alive was a definite plus.

"Families with grandmothers alive were larger by about two and the survival of these grandchildren to age 15 was much improved."

This beneficial effect was only seen when the matriarchs lived nearby. Which suggests that grandmothers help by playing an active role in their grandchildren's lives. Unfortunately, that role is tougher for them to fulfill⁵ as they get older. Which brings us to the second study.

Researchers at the University of Turku in Finland used church records from the 18th and 19th centuries. They found that the benefits associated with having a grandmother on hand depended on her age. Once nanna hit 75, the grandchild survival benefit disappeared—and then some.

"In other words, it was better for grandchildren to have no living grandmother at all than it was to live with an old one or one that was in poor health."

Simon Chapman, a doctoral student in biology.

"This was almost certainly due to some form of indirect resource competition, though, rather than wickedness on the part of co-resident grandmothers."

So a healthy grandma helps make for a healthy grandchild. If "over the river and through the woods to grandmother's house" isn't too long a trip.

Thanks for listening for Scientific American — 60-Second Science. I'm Karen Hopkin.