You probably don’t need to look at those credit-card bills from Christmas to know that kids are expensive.

As of 2013, the cost of raising a child in the United States (not including college) was a quarter of a million dollars. That easily doubles when tuition is thrown in. And never mind that we live in the most expensive region in the country. It’s why more and more observers of wealthy families in New York say that a third child is a “status symbol” and large families are “luxuries.”

But what if having more children is bad for the children?

A new National Bureau of Economic Research study from Chinhui Juhn and C. Andrew Zuppann at the University of Houston and Yona Rubinstein of the London School of Economics looked at data from the National Longitudinal Survey of Youth in order to learn how the addition of more siblings affected children’s math and reading abilities, behavioral issues and home environments.

On the whole, the news wasn’t good. Behavioral problems increased and cognitive scores fell. And the number of years of education each child received also dropped.

Perhaps these results should have been obvious to anyone who has looked at the American family since we stopped living on farms and using child labor. The 20th century marked the first time in human history that having more children didn’t increase your economic standing.

Having more able-bodied boys and even girls used to mean that more people in your household could bring in income. And since child-mortality rates were so high, people tried to have more kids.

But now children are an expense. We invest in our children, buying them nice clothes and good educations, in the hopes that they’ll carry on our ambitions and values after we’re gone.

Now many Americans have come to the decision that they should expend more on each child rather than divide the pie into smaller pieces. It’s not just a microeconomic problem. Falling birthrates mean we’re a less-productive country and also less able to sustain welfare-state programs like Social Security.

But they also change our social fabric in ways that are hard to measure. What does it mean that fewer children grow up with siblings or cousins or aunts and uncles? How does it affect our communities when families begin to shrink?

Before putting that crib out on the curb, though, it’s worth reading an important corollary to the NBER study: The harmful effects of more kids in a family don’t hold true for women with higher levels of education.

When the researchers divided families by the mother’s score on the Armed Forces Qualifying Test, they found “strikingly different results . . . For children with mothers with below median AFQT scores, the arrival of younger siblings have large and significantly negative effects on cognitive skills while for children with mothers with above median score, the effects are much smaller and not significant.”

The reasons should be obvious: “Mothers with high AFQT scores are more likely to leave employment and reduce hours worked following the birth of younger siblings.”

In other words, families where mothers are more educated can offer more maternal attention to children. On the one hand, this should suggest that having more children is not in and of itself a problem for kids. And despite all of the pressures from environmentalist loons and zero-population-growth fanatics, we needn’t reduce our fertility below the replacement rates they’re currently at.

On the other hand, it makes worse a trend we’ve seen for the past few decades — family inequality.

Families on the lower end of the economic spectrum have already suffered because of low marriage rates and higher incidences of out-of-wedlock childbirth. Now we know that children from poor families, where birth rates tend to be the highest, also have even more challenges.

There’s no easy fix. When it comes to having the resources to devote to multiple children, two parents are no doubt better than one. And though we may (as the authors do) speculate about the benefits of government-sponsored childcare, there may be no substitute for maternal attention.

Naomi Schaefer Riley is a senior fellow at the Independent Women’s Forum.