Women are more likely to argue with their mothers-in-law after they have children and even more likely to start sh*t when grandma babysits regularly, according to a study of 1,200 Finnish couples. The study suggests that familiarity ultimately breeds contempt. When we see our in-laws as family — with common progeny and shared responsibilities — we’re more likely say we hate our in-laws and want to kill them.

“Daughters-in-law were more likely to report conflicts when their mother-in-law provided more grandchild care,” study co-author Mirkka Danielsbacka said in a statement. “This indicates that the increase in conflicts between in-laws are related to grandchild care.”

There are literally troves of mother-in-law jokes (What’s the difference between in-laws and outlaws? Outlaws are wanted!) and for good reason. In-laws can make the happiest marriage unpleasant. Just ask Terri Orbuch, Ph.D., a sociologist who tracked nearly 400 marriages over a period of 26 years and found that relationships with in-laws are closely correlated with risk of divorce.

In an effort to figure out why relationships with our in-laws tend to sour over time, Danielsbacka and colleagues asked 1,202 married Finnish men and women how often they had blow-out fights with their parents and in-laws. In general, Finns reported getting into more fights with their parents than their in-laws…until they had kids.

Once grandkids entered the picture, participants continued to fight with their own parents at roughly the same rate as before but began to argue with their in-laws, as well. Fights between daughters-in-law and mothers-in-law were even more common when the grandparents helped raise the children or visited often, suggesting that mothers-in-law who want relationships with their grandchildren really can’t win.

“The shared reproductive interest that is created through a grandchild…provides new reasons for grandparents to influence and interfere in the lives of other family members, which in turn may be reflected in conflict proneness,” the authors write.

Part of the spike in conflict likely comes from the fact that in-laws are around more often once grandkids are in the picture. Regular, unwanted visitors can turn any cordial relationship into a battleground. “Conflicts were related to higher contact frequencies,” the authors wrote.

But there’s also a more comforting explanation. Danielsbacka and colleagues raise the possibility that we don’t fight with our in-laws until we see them as family — and we don’t see them as family until we give birth to their grandkids. Evolutionary psychologists call this a “kinship penalty.” Until you feel a kinship with someone, the theory goes, it’s hard to hate them. And until your in-laws feel like parents, it’s hard to fight with them like you would fight with your own parents.

So the next time you want to push your mother-in-law off a cliff, take a deep breath. She probably just wants to see her grandkids. And you probably just want to call her “mom.”

This article was originally published on Aug. 8, 2017