Demographers have documented steep drops in fertility rates among illiterate Bulgarian peasants, for example, and stable high ones in industrial England. In the United States, birth rates began to drop in a period when huge majorities still lived in rural areas, women lacked independence, secondary education was rare, churches were strong, contraceptives were rudimentary, child mortality was high (about 20 percent), and nonfamily farm labor did not come cheap. After World War II, fertility climbed in the United States—though not because Americans suddenly desired large families; the Baby Boom was a result of more Americans deciding to have small or medium-size families rather than none at all. To this day, no one has succeeded in writing a formula for higher or lower fertility: There is no single explanation, only possibilities of varying likelihood.

Japan’s birth rate may be falling because there are fewer good opportunities for young people, and especially men, in the country’s economy. In a country where men are still widely expected to be breadwinners and support families, a lack of good jobs may be creating a class of men who don’t marry and have children because they—and their potential partners—know they can’t afford to. (..)

In a culture that places such an emphasis on men being breadwinners, this has serious implications for marriage and childbearing. Men who don’t have regular jobs are not considered desirable marriage partners; even if a couple wants to get married, and both have irregular jobs, their parents will likely oppose it, according to Ryosuke Nishida, a professor at Tokyo Institute of Technology who has written about unemployment among young workers. About 30 percent of irregular workers in their early 30s are married, compared to 56 percent of full-time corporate employees, according to Kingston. “Japan has this idea that the man is supposed to get a regular job,” said Nishida. “If you graduate and you don't find a job as a regular employee, people look at you as a failure.” There’s even a tongue-in-cheek Japanese board game, Nishida told me, called “The Hellish Game of Life,” in which people who don’t land a regular job struggle for the rest of the game.

Women seeking full-time work frequently find themselves in irregular jobs too, which also has implications for raising a family, since the hours are unpredictable and the pay is low. But it is more of an obstacle for marriage if a man doesn’t have a good job—roughly 70 percent of women quit working after they have their first child, and depend on their husband’s salary for some time.

Women in Japan’s big cities say they’re getting tired of the lack of available men. While in Tokyo, I visited an event put on by Zwei, (..) matching women in Japan’s big cities with men in other areas of the country, where men are more likely to have good jobs and be considered viable partners. “Men in this city are not very masculine and they don't want to get married,” Kouta Takada, a Zwei staff member, told me. A recent survey of Japanese people aged 18 to 34 found that nearly 70 percent of unmarried men and 60 percent of unmarried women aren’t in a relationship.

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.

Read: A Surprising Reason to Worry About Low Birth Rates

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.  

Finally, childless cities exacerbate the rural-urban conundrum that has come to define American politics. With its rich blue cities and red rural plains, the U.S. has an economy biased toward high-density areas but an electoral system biased toward low-density areas. The discrepancy has the trappings of a constitutional crisis.