I have four kids. I don’t strike people as the type to have so many, nor does my wife. We’re professors. Neither of us is conventionally religious. We spent our 20s in Brooklyn as vegetable-blending free spirits. I drive a Prius on principle, even though I’m 6 foot 8 and my head hits the ceiling.

It’s hard to say how we ended up with such a large family. When people ask, I say (1) my wife likes babies, (2) I tend to assume I won’t regret having another child, and (3) we love the kids we have. But there’s an element of mystery, even to me. Any answer feels incomplete. Maybe that’s because the fuller explanation is buried too deep, in layers of instinct and social expectation. I think it’s hard for people to say exactly why they have kids or not—and if they do, how many.

Even trickier is the question of why birth rates rise and fall across huge groups. Until recently, demographers worried mostly about overpopulation. Now roughly half the world’s people live in countries with “below replacement” fertility rates, and that proportion is growing. The environmental benefits of this trend are obvious. But low birth rates also threaten welfare states with bankruptcy, and nations with the destabilizing politics of cultural extinction.

As demographic anxiety goes global and populist, a roiling debate is forming around basic questions: Why do some people want children, while others do not? Why do some societies seem to be shrinking? Can a progressive, reproductive-freedom-embracing society survive over time? Or is it doomed to a slow, comfortable death?

It’s impossible to address these questions without taking a long view.

The United States was among the first modern nations to see a steady, large-scale fertility decline. In 1800, the average American had seven kids. By 1900, that figure was 3.5, and President Theodore Roosevelt was excoriating his people for committing “race suicide”—a “sin for which the penalty is national death, race death; a sin for which there is no atonement.”

Roosevelt’s denunciations caused a sensation, not only because the president was discussing sex rather than, say, tariffs, but also because his diagnosis was so grim. A major civic debate broke out. In letters, newspapers, radio shows, and surveys, thousands of Americans looked to themselves and their communities, and tried to explain the desire for smaller families. Over the next few decades, they produced a highly plausible composite portrait of the popular birth-control movement, with birth control here meaning the practice of controlling births, rather than specific technologies used for that purpose.

Observers considered a thick tangle of overlapping social explanations, ranging from sprawling generalities to daily practicalities. At the practical end, for example, was housing discrimination against large families. Early 20th-century landlords had both the right and the inclination to turn away tenants based on the number and the unruliness of their children. Landlords posted signs prohibiting “dogs and children”; in 1904, stories circulated about a mother of five from Brooklyn who was turned away by 87 apartment houses, despite her husband’s steady job.

Somewhat less practical were a series of aspirational issues. Americans seemed to expect more for their children and themselves than did previous generations. More education, better health, less grueling work, more leisure. On top of that, many parents shared a growing sense that responsibility for their children’s success fell on them more than on the child.

And then there were economic considerations. Everyone agreed that children were expensive—even farmers, for whom a child was only truly useful for a short time, as a teenager, before he left home. Crucially, though, economic decisions about children were unlike economic decisions about lumber. Children’s costs and benefits were bound up in fundamental moral questions about self, society, transcendence, and cosmic time. Am I born in reproductive debt to my family or community? Is birth control basically selfish, serving irresponsible adults, or altruistic, serving vulnerable children? Am I part of a transcendent chain of being that stretches far into past and future, or the sovereign of my present?

In other words, the economic rationality of having zero children was never in doubt. The key question was where to draw moral-economic lines between “luxury” and “prudence.” How much money did each child require? Answers to this question varied wildly.

Americans tended to agree, however, that a second key factor often determined these judgments: a person’s ideas about God and nature. What was essential, though, was not whether someone went to church every Sunday or belonged to one denomination or another. Plenty of devout parishioners had small families, notwithstanding priestly or divine injunctions to “be fruitful and multiply.”

More important was whether someone’s understanding of God or nature caused him to focus, more than other people, on the mystical, eternal, and cosmic rewards of children, rather than their present and material costs. When children served as connections to infinite time and space, and escapes from mortal suffering and decay, people were more likely to have them in any number. This viewpoint required no piety or acceptance of religious dogma. An agnostic Darwinist could find higher purpose in contributing to an eternal biological continuum, as ordained by Nature. A mystic could marvel at the miracle of new life. For better or worse, children’s costs were less likely to daunt people who valued modesty before the cosmos, no matter how they saw that cosmos.

Finally, there was one meta-justification for birth control that brought together the others in a dense, actionable nugget: “modern life” or “the times.” For Americans asking themselves whether to have two children or three, or any children at all, modernity was a bleeding reality, not an academic abstraction. The demand for smaller families seemed to spring from history itself—from the faster, riskier, more open-ended world that had replaced the “old-fashioned” world of one’s parents, grandparents, and ancestors. Rationality (or greed), pragmatism (or faithlessness), mastery of fate (or hubris): Americans disagreed passionately about whether birth control was part of the good or the bad in modern life, but they agreed that history itself seemed to demand ever-tighter control over fertility. The best-adapted people always seemed to have the smallest families.

The idea that the olden days were friendlier to families is hardly airtight as empirical history. 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.

Still, the “new times” narrative was undeniably a powerful motivator. Measurable indicators of modernity might be much lower in Missouri than in Manhattan, but if Missourians believed staying abreast of the times demanded greater control over fertility, they might well exercise that control.

“Race suicide” was a powerful idea—not just because many Americans were reflexive white-Protestant supremacists, but also because virtually all Americans took solace in the idea that enduring continua such as family, community, and culture lent meaning and dignity to fleeting lives. “Being modern” was an even stronger one.

Birth control and modernity remain closely associated today, in the United States and abroad. Each is a cause of the other, an effect of the other, and a buttress to the other’s moral authority. The freer, richer, and more “advanced” the place or people, the smaller the families. The smaller the families, the more developed the place. “Development” is movement toward the sort of liberal society where people build lives less around service to the immortal tribe and more around the present well-being of individuals.

As below-replacement fertility spreads around the world, however, this association could become less mutually advantageous. Particularly if modern standard-bearers such as Japan or Germany continue recording decade upon decade of ultralow fertility, birth control and modernity could begin to seem more like interconnected problems than obvious developmental goals.

Already there are signs that local low fertility is becoming a folk issue in much the same way that global high fertility became one during the “population bomb” decades of the late 20th century. In countries with the longest records of low fertility, new fears of race suicide are fueling well-known populist and ethno-nationalist movements.

Just as significant, though, is awareness of these issues outside the lowest-fertility countries, where people who admire developed societies, and may wish to migrate to them or build them in situ, are unlikely to view these trends positively. When “miracle” nations such as South Korea, for example, announce that their fertility rates have fallen below one, it may alarm Koreans, but may also serve as a cautionary tale about the Korean model.

Immigration is an obvious remedy for low-fertility societies with shrinking workforces. But immigrants from high-fertility countries tend to quickly adopt receiving countries’ smaller-family norms—if not in the first generation, then in the second. This leaves the receiving country’s age structure largely intact over time, so that demand for new working-age immigrants continues unabated. The resulting prospect of indefinite large-scale in-migration should not pose a major political problem, perhaps, but it does, especially in places where a majority possesses a strong sense of indigeneity.

One approach to these issues is to do nothing and celebrate the fact that falling global fertility (1) is good for the planet and (2) reflects and promotes unprecedented human freedom and flourishing. Both these points are true, but they do not address the fact that most people identify primarily with groups smaller than the species and with places smaller than the planet.

Another approach is to enact laws that attempt to reconcile the demands of parenthood with those of modern market economics. People in low-fertility countries tend to have fewer children than they want, partly for economic reasons, and policies such as subsidized child care, state-mandated parental leave, and even direct cash payments can help make children more feasible. So can harder-to-legislate goals such as stabilizing youth employment and getting men more involved in child-rearing. Pro-family policies are increasingly popular with politicians left, right, and center.

Laws, however, can work only at the margins. Birth control has always been a radically social social movement, composed of small people acting on big ideas. All adults must make decisions about reproduction, and those decisions feel important, but they are very difficult for leaders to police or influence.

Over the long term, liberal societies are not equally well served by very low birth rates as by near-replacement ones. That is partly because the perception that liberalism is “dying” could become self-fulfilling, with both insiders and outsiders abandoning the liberal social model. It is also because the greatest tangible reward most liberal societies offer, the promise of material prosperity and political stability, can erode in countries where a dwindling supply of workers struggles to bankroll a heavily indebted welfare-and-eldercare state, and where citizens confront periodic eruptions of nativism.

The German philosopher-prophet Oswald Spengler believed that “the sterility of civilised man … is not something that can be grasped as a plain matter of Causality … It is to be understood as an essentially metaphysical turn towards death.” For Spengler, modernity was a passing phase, a short and riotous orgy before the fall. His dark vision inspired prominent Nazis. But an obscure American contemporary of Spengler, the sociologist Delos Wilcox, saw the situation more fluidly. “The purpose of reproduction is the renewal and improvement of human life, on the assumption that life is worth living when it is lived well,” he wrote. Apart from that assumption, “what reason have I to assume responsibility for the perpetuation of life?”

Versions of this question are becoming part of the fabric of our era. If indeed free and egalitarian societies don’t reproduce themselves over time, that outcome may ultimately be taken as a just verdict on the desirability of human life as we live it. More likely, some new idea will arise among us to dignify and eternalize our lives and way of life. That new world could retain the best of our blessings.

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