If you live with your mother-in-law or your own mum, you might have fewer children. That's according to a new study, which looked at the impact of a live-in grandma on women's fertility.
As part of the research, published in the journal Royal Society Open Science, evolutionary anthropologists from the University of Vienna examined the medical records of over 2.5 million women aged between 15 and 24 from 14 countries around the world. And the results were illuminating, to say the least.
Couples where mums or mother-in-laws lived under the same roof had fewer kids than in those where couples lived alone.
"The presence of any mother in the household is invariably associated with a significantly lower number of children compared to women living only with their spouse," the authors write in their paper. Perhaps surprisingly, however, having a mother-in-law on board has less of an impact on fertility than a woman's own mum. "In most countries, a woman's number of children is lower if she lives with her own mother as compared to her husband's mother in the household," the authors note.
Describing the findings as "interesting", the researchers argue, "one would expect that both mothers should have 'some biological interest' in a higher number of grandchildren."
But is it really that common to have a live-in grandmother? It all depends on the country. The researchers note that worldwide, the most common living arrangement was for women to reside with their spouse only. In Iraq, over half of women (53 per cent) live with their mother-in-laws, followed by Pakistan at 41 per cent. In the US, only 1.5 per cent of mums reside with their mother-in-law. Australian census data was not included in the study.
Around the world, living with a woman's own mother was the least common household setup, ranging from less than one per cent in Malawi to 17 per cent in Thailand. Younger women were more likely than older women to reside with either their own mum or their mother-in-law.
But why might having a grandma under the same roof impact a woman's fertility? Keeping in mind that the researchers are evolutionary anthropologists, the team suggest a number of different explanations - including the fact that grandmothers may "slow down the woman's reproduction". "Both grandmothers may foster a 'slower life-history strategy' in terms of investing more in a smaller number of children," they note.
But slowing down a woman's reproduction is not their only theory. "An alternative explanation would be that the presence of any mother in the household does not affect a woman's number of children," the authors write, "but that women who start childbirth at very young ages have both a higher probability to live with a mother in the household and to have lower overall fertility."
The team highlight that socioeconomic factors may also be at play under these circumstances. For example, women living under the same roof as their own mum or or their mother-in-law may be facing a "a difficult and complicated stage of life", such as health issues or unemployment, which in turn may have an impact on how many children they have.
While it's important to note that the findings are correlational and can't prove a causative effect of grandma on a woman's fertility, they're certainly worth a ponder. Sure, it may take a village to raise a child, but it might be better to have the village move in after you've finished reproducing - rather than before.