Letting your mother-in-law move in may not be the best way to grow your family, a new study suggests. That’s surprising, because experts generally assume that humans are one of the few species with grandmothers (that is, post-menopausal females with long lives) because that confers an evolutionary advantage—idle hands, available to help new mothers manage their brood so they can raise larger families. But whether grandmas are evolving away from their intended purpose or whether the Grandmother Hypothesis was simply wrong to begin with, this new study on two million families worldwide suggests that, when mom moves in, couples tend to have fewer children.
“It would make sense that a woman receives help by her mother or her mother-in-law,” study coauthor Martin Fieder, an anthropologist at the University of Vienna, told Fatherly. “However, this very reasonable theoretical concept has been challenged by less clear data.”
One study of the Krumhörn Population that lived in Germany between 1720 and 1874 found that children who lived with both their mothers and grandmothers were less likely to survive into adulthood. Now, those findings seemed to be the exception rather than the rule (many, many, many other studies have shown that active grandmothers increase the likelihood of grandchildren surviving) and it seems that children do benefit from having their grandparents around. But the notion that grandmothers should live with their grandchildren is not necessarily part of the Grandmother Hypothesis. Besides, alternate theories behind menopause have been proposed. Perhaps, as the Mother Hypothesis suggests, women cease reproduction so that they can better care for their existing children. Perhaps there’s no evolutionary advantage to menopause at all.
For this new study, Fieder and colleagues examined data on 2,478,383 married women from 14 different countries. With the exception of women in Iraq, they found that it was not the norm for grandmothers to live with their grandchildren. But across cultures and borders, one trend stood out—when paternal or maternal grandmothers do live in the same house as their children and grandchildren, they end up with fewer grandkid. The knee-jerk reaction is that, since making babies requires sex, a live-in mother-in-law ruins a dad’s game. But it’s not that simple.
“We actually have no ideas on that,” Fieder says.
What researchers can say is that the results likely have to do with either resource competition, reproductive competition, or some combination thereof. Interestingly, Fieder and colleagues did find evidence grandmothers had a slowing-down effect on reproduction—mothers who lived with their children’s grandmothers reproduced earlier in life, but had fewer children overall.
“As we know that giving birth to children on short inter-birth intervals, means also having a higher risk of children’s death, slowing down reproduction may help to keep more children alive,” Fieder speculates. He adds that the findings are also limited by the fact that not all the women were finished having children at the time of the study and by the fact that the team did not control for multiple marriages (so some grandmothers were likely not related to all of the grandchildren).
Which means that the findings, though significant, probably shouldn’t affect your day-t0-day parenting decisions. “Take scientific results as interesting, but live your life such as seems most appropriate for you,” Feider says. And if that means Grandma is staying in the guest room, so be it.