New York is the poster child of this urban renaissance. But as the city has attracted more wealth, housing prices have soared alongside the skyscrapers, and young families have found staying put with school-age children more difficult. Since 2011, the number of babies born in New York has declined 9 percent in the five boroughs and 15 percent in Manhattan. (At this rate, Manhattan’s infant population will halve in 30 years.) In that same period, the net number of New York residents leaving the city has more than doubled. There are many reasons New York might be shrinking, but most of them come down to the same unavoidable fact: Raising a family in the city is just too hard. And the same could be said of pretty much every other dense and expensive urban area in the country.

In high-density cities like San Francisco, Seattle, and Washington, D.C., no group is growing faster than rich college-educated whites without children, according to Census analysis by the economist Jed Kolko. By contrast, families with children older than 6 are in outright decline in these places. In the biggest picture, it turns out that America’s urban rebirth is missing a key element: births.

Cities were once a place for families of all classes. The “basic custom” of the American city, wrote the urbanist Sam Bass Warner, was a “commitment to familialism.” Today’s cities, however, are decidedly not for children, or for families who want children. As the sociologists Richard Lloyd and Terry Nichols Clark put it, they are “entertainment machines” for the young, rich, and mostly childless. And this development has crucial implications—not only for the future of American cities, but also for the future of the U.S. economy and American politics.

The counties that make up Los Angeles, Chicago, New York City, and Philadelphia shed a combined 2 million domestic residents from 2010 to 2018. For many years, these cities’ main source of population growth hasn’t been babies or even college graduates; it’s been immigrants. But like an archipelago of Ellis Islands, Manhattan and other wealthy downtown areas have become mere gateways into America and the labor force—“a temporary portal,” in the words of E. J. McMahon, the founder of the Empire Center for Public Policy. “The woman from Slovakia comes to Queens, lives in her second cousin’s basement, gets her feet on the ground, and gets a better apartment in West Orange, New Jersey,” he said. Or a 20-something from North Dakota moves to Chicago after school, works at a consultancy for a few years, finds a partner, and moves to Missoula.

But if big cities are shedding people, they’re growing in other ways—specifically, in wealth and workism. The richest 25 metro areas now account for more than half of the U.S. economy, according to an Axios analysis of government data. Rich cities particularly specialize in the new tech economy: Just five counties account for about half of the nation’s internet and web-portal jobs. Toiling to build this metropolitan wealth are young college graduates, many of them childless or without school-age children; that is, workers who are sufficiently unattached to family life that they can pour their lives into their careers.

Cities have effectively traded away their children, swapping capital for kids. College graduates descend into cities, inhale fast-casual meals, emit the fumes of overwork, get washed, and bounce to smaller cities or the suburbs by the time their kids are old enough to spell. It’s a coast-to-coast trend: In Washington, D.C., the overall population has grown more than 20 percent this century, but the number of children under the age of 18 has declined. Meanwhile, San Francisco has the lowest share of children of any of the largest 100 cities in the U.S.

The modern American city is not a microcosm of life but a microslice of it. It’s becoming an Epcot theme park for childless affluence, where the rich can act like kids without having to actually see any.

Okay, you might be thinking, but so what? Happy singles are no tragedy. Childlessness is no sin. There is no ethical duty to marry and mate until one’s fertility has exceeded the replacement rate. What’s the matter with a childless city?

Let’s start with equity. It’s incoherent for Americans to talk about equality of opportunity in an economy where high-paying work is concentrated in places, such as San Francisco and Manhattan, where the median home value is at least six times the national average. Widespread economic growth will become ever more difficult in an age of winner-take-all cities.

But the economic consequences of the childless city go deeper. For example, the high cost of urban living may be discouraging some couples from having as many children as they’d prefer. That would mean American cities aren’t just expelling school-age children; they’re actively discouraging them from being born in the first place. In 2018, the U.S. fertility rate fell to its all-time low. Without sustained immigration, the U.S. could shrink for the first time since World World I. Underpopulation would be a profound economic problem—it’s associated with less dynamism and less productivity—and a fiscal catastrophe. The erosion of the working population would threaten one great reward of liberal societies, which is a tax-funded welfare and eldercare state to protect individuals from illness, age, and bad luck.

This threat sounds hypothetical, but low fertility rates are already roiling Western politics. In a 2017 essay, I explained how low fertility in the U.S. and Europe might be feeding into right-wing populism. The theory went like this: Low natural population growth encourages liberal countries to accept more immigrants. As growth stalls, native-born low- and middle-class workers become frightened of the incipience of foreign workers. To protect themselves, the white petit bourgeoisie turns to retrograde strongmen who promise to wall out foreigners.