Want Just a Few Kids? Ask Mom to Move In

mother and daughter with son mother and daughter with son
Women whose mothers live with them later in life have fewer children than women whose mothers live elsewhere. Supachok Pichetkul/EyeEm/Getty Images

Your mother and mother-in-law might may want more grandchildren, but if either of them lives with you, they might also be acting as a form of birth control.

That's right. According to a new report from the Department of Anthropology at the University of Vienna, women living with their own mothers or their mothers-in-law have fewer children than women who live only with their spouses. Until these results, evolutionary biologists had just assumed the opposite was true.

In the study, published in Royal Society of Open Science, anthropologists Martin Fieder, Susanne Huber and Patricia Zahourek analyzed census data of nearly 2.5 million married women across a broad spectrum of cultures. The women, aged between 15 and 34 years, lived in 14 countries, including the U.S., Iraq, Malawi, Thailand and Pakistan. In all cases, the husbands lived at home, as well.

One interesting finding is women had less children most often when their own mothers lived in their household. Couples who live with their husband's mother had slightly more children, on average, but still fewer than couples who lived on their own.

The study considered variables such as the education level, reproductive time span, employment, and urban and rural living conditions. The researchers conducted the survey to test a commonly held notion — one found in previous studies — that the presence of a grandmother in the home may positively affect a woman's fertility.

Among one possible cause for the newfound results considered by the study's authors: competition for resources among family members. "In three-generation households, grandparents are not only providers of support but can also be resource competitors," the study said.

In other words, grandma needs to eat, too, and have a place to sleep — which could mean less food for children, and fewer bedrooms to sleep in (if any at all), which could ultimately translate to couples having less children.

In short, the next time your live-in mother-in-law demands more grandchildren, you might want to consider moving her to her own apartment.