Seriously, why aren’t millennials having kids?

Every two years, the Census Bureau quietly appends a battery of fertility-related questions to its workhorse monthly questionnaire, the Current Population Survey, our go-to source for everything from the unemployment rate to Americans’ moving habits. It’s smaller than the immense annual American Community Survey, but it’s one of the few major surveys that asks how many times American women have given birth.

As we analyzed the latest figures, from 2022, our brains spun in our skulls: Since the mid-1980s, the rate at which we produce only children has remained absolutely flat. Something like 1 in 5 American women ages 25 to 44 are one and done.

That’s bizarre, given birthrates! But let’s zoom out and look at the whole universe of possible family sizes.

First, we noted that families with three or more kids plunged in the 1980s, as birth control, education and greater opportunity helped women pile into the workforce. That’s also when only children rose to their current level. Families shifted again after the Great Recession when, among women 25 to 44, even having two children lost its luster. The number of women who had zero children soared. Only children held steady.

To be sure, this is partly because women are starting their families later and thus having second children later. But even among women in their early 40s, the share of only children has barely budged in more than three decades, crawling from 17 percent in 1990 to 19 percent in 2022.

That suggests a simple explanation: If people want kids, they want more than one. A consistent minority stops at one, be it for biological, philosophical or logistical reasons. But otherwise children seem to be a multiple-or-nothing proposition.

Our friends at Gallup confirmed this. A poll this summer found that almost nobody — just 3 percent of Americans — considers one child to be the ideal family size.

Even if you poll people who currently have one child, only 6 percent of them consider one to be the loveliest number. Zero is even less popular.

About three quarters of us think two (44 percent) or three (29 percent) children would be ideal. And the parents who have that many kids are much more likely to have the precise number of kids they think is ideal. That contrasts sharply with parents with fewer kids, who almost always think more would be ideal, and parents with four or more kids, about half of whom think the ideal family is probably smaller than theirs. Whoops.

University of Texas psychologist Toni Falbo has studied only children such as herself since the mid 1970s, when they were legitimately rare. She told us that (largely false) stereotypes have led parents to believe that having just one child amounted to “mistreating your child by not providing them with another sibling.”

There’s evidence that only children may be more likely to divorce than people from large families, and may have higher body mass indexes in adolescence, Falbo told us. But she’s found little backing for the persistent belief that only children struggle socially, especially after kindergarten.

“It turns out only children are not more selfish than others. They aren’t lonelier than others,” Falbo said. “Ironically, in many ways, they’re less lonely than other people because they’re accustomed to being alone.”

So why are people choosing none over one? The biggest determinants of childlessness seem to be youth, marriage (or lack thereof) and higher education. The shift toward zero kids came fastest among younger women, especially those in their 20s, though we now see it across the age spectrum.

But look deeper at that data and you’ll spot something wild. Women in their early 20s embraced childlessness first, with a sharp rise beginning around 2002. That happens to be when the first millennials, born in 1981, entered that age group. For women in their later 20s, the jump in childlessness happened in 2006, just as the first millennials arrived.

As you ascend the age spectrum, the millennial echo follows. When the oldest millennials hit their 40s, even 40-year-olds become more likely to go childless.

Generations are squishy, man-made distinctions. Outside the baby boom, it’s rare to see such a vivid generational turning point. We’d love to have a perfect explanation for it. Send us your theories!

For now, we can tell you that marriage rates have steadily declined, and unmarried folks are less likely to have kids. The same goes for that other major marker of building your own household: a building. Millennials were late to homeownership, which made it harder to start families.

But neither offers a full explanation. Both married and unmarried Americans have shifted toward childlessness in the millennial era. And levels of childlessness have actually accelerated among millennials as their homeownership rates have recently grown.

Instead, just about every source we consulted pointed to the broader economic climate. Hammered by the Great Recession, soaring student debt, precarious gig employment, skyrocketing home prices and the covid-19 crisis, millennials probably faced more economic headwinds in their childbearing years than any other generation. And, as sociologist Karen Benjamin Guzzo, director of the Carolina Population Center at the University of North Carolina, told us, it put them behind on everything you’re supposed to line up before you have kids.

“We have a pretty strong set of prerequisites: You absolutely should finish school, and have a decent job, and you should make a decent income, and you should be in a good partnership, and you should live on your own,” Guzzo told us. “That takes a while to accomplish, especially in this day and age. Some people may feel like they’re never going to be in a good place.”

And, unlike previous generations, millennials had the means to delay pregnancy thanks to affordable, long-acting birth-control options, said Alison Gemmill, a demographer at the Johns Hopkins Bloomberg School of Public Health.

“Historically, one of the reasons why we think the U.S. has had such a high fertility rate compared to other countries was related to unintended and unwanted pregnancies that resulted in births,” Gemmill told us. Now that it’s easier to avoid accidental pregnancies, more women are having kids later, or not at all.

Sociologist Sarah Hayford directs Ohio State’s Institute for Population Research and has, with Guzzo, studied the living daylights out of another key data source, the National Center for Health Statistics’ National Surveys of Family Growth. She thinks millennials may still come around on kids.

“A big part of the uptick in childlessness is delay rather than permanent childlessness,” Hayford told us. “Even among women in their thirties, a lot will go on to have a child.”

If women are able to follow through on their delayed family plans, much of the rise in childlessness could be erased, according to a 2020 analysis of the same data set by Gemmill and Caroline Sten Hartnett of the University of South Carolina. But with older millennials in their 40s, time for a reversal may be running out.

When that almost mythical perfect time to have children does arrive, some women will find they don’t have the means. “In the United States, we overestimate our ability to get pregnant later or how readily available medically assisted reproduction might be,” Guzzo said. “I mean, it is available, but it’s crazy expensive. Your average person can’t afford it. It’s often not covered by insurance.”

Meanwhile, a 2021 poll from our friends at Pew Research found that about 44 percent of childless adults ages 18 to 49 said they were not too likely, or not at all likely, to have children, a sharp increase from the 37 percent who said the same thing in 2018.

And a hefty 56 percent said they just didn’t want kids. (The remainder pointed to medical reasons, financial reasons and lack of a partner. The state of the world and climate change came in a distant fifth and sixth out of seven.)

When we asked about childless couples in an earlier column, many readers pointed to the rise of same-sex marriage. But the latest Census Bureau estimates, from 2021, show that fewer than 1 in 100 U.S. households are led by same-sex couples. Even if none of those couples had kids, an absurd assumption, it wouldn’t come close to explaining the rise in childlessness.

After consulting our own household — which notably includes a brilliant dog but zero children — we began to wonder if it may have something to do with millennials being the first generation that isn’t likely to do better than their parents, according to Opportunity Insights. A recent WSJ/NORC poll found that 78 percent of Americans aren’t confident their children will have better lives than they did.

We have little data to back up this speculation, but many of our sources wholeheartedly agreed. About 90 percent of kids born in 1944 outdid their parents; even negligent mothers and fathers could produce a surefire success. For kids born in 1984, that number was just 50 percent. These days, when the outlook may be even bleaker, there’s intense pressure to pump your kids up with every available ounce of organic superfood, superior schooling and extracurricular enrichment to give them a slim shot at getting ahead.

So the decision to avoid having children may amount to a kind of performance anxiety in the face of intense expectations and weak governmental and social support, Guzzo said: “If I don’t do everything right, then my kid will end up living on my couch forever or be a serial killer. … I don’t know if or when I’ll have what it takes to be a ‘good’ parent.

“The stakes are so high. I don’t want to screw it up.”

Ahoy there! The Department of Data needs your queries. Who’s most likely to live with their adult siblings? Where are hailstorms getting more dangerous? What did the stress caused by the Beltway sniper do to expectant mothers in the region? Just ask!

If your question inspires a column, we’ll send an official Department of Data button and ID card. This week, we’ll make the trek all the way across the newsroom to deliver a button to Herman Wong, The Post’s deputy general-assignment editor … or at least we would if he wasn’t out on paternity leave with his first — and so far only! — child.