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It was a girl.

On Sunday, October 30, 2011, just before midnight, Danica May Camacho entered the world in a crowded Manila hospital, bringing the human population of our planet to seven billion. Actually, the scales could have tipped a few hours later, in a village in Uttar Pradesh, India, with the arrival of Nargis Kumar. Or it might have been a boy, Pyotr Nikolayeva, born in Kaliningrad, Russia.

Of course, it was none of them. The birth that took us to seven billion people was attended by no cameras and ceremonial speeches because we can never know where or when the event occurred. We can only know that, according to the United Nations’ best estimates, we reached seven billion sometime around October 31 of that year. Different countries designated certain births to symbolize this landmark in history, and Danica, Nargis, and Pyotr were among those chosen.

For many, there was no reason to celebrate. Indian health minister Ghulam Nabi Azad declared that a global population of seven billion was “not a matter of great joy, but a great worry. . . . For us a matter of joy will be when the population stabilizes.” Many share Azad’s gloom. They warn of a global population crisis. Homo sapiens is reproducing unchecked, straining our ability to feed, house, and clothe the 130 million or more new babies that UNICEF estimates arrive each year. As humans crowd the planet, forests disappear, species become extinct, the atmosphere warms.

Unless humankind defuses this population bomb, these prophets proclaim, we face a future of increasing poverty, food shortages, conflict, and environmental degradation. As one modern Malthus put it, “Barring a dramatic decline in population growth, a rapid decrease in greenhouse gas emissions, or a global outbreak of vegetarianism—all of which are trending in the opposite direction at the moment—we’re facing nothing less than the end of plenty for the majority of the earth’s people.”

All of this is completely, utterly wrong.

The great defining event of the twenty-first century—one of the great defining events in human history—will occur in three decades, give or take, when the global population starts to decline. Once that decline begins, it will never end. We do not face the challenge of a population bomb but of a population bust—a relentless, generation-after-generation culling of the human herd. Nothing like this has ever happened before.

If you find this news shocking, that’s not surprising. The United Nations forecasts that our population will grow from seven billion to eleven billion in this century before leveling off after 2100. But an increasing number of demographers around the world believe the UN estimates are far too high. More likely, they say, the planet’s population will peak at around nine billion sometime between 2040 and 2060, and then start to decline, perhaps prompting the UN to designate a symbolic death to mark the occasion. By the end of this century, we could be back to where we are right now, and steadily growing fewer.

Populations are already declining in about two dozen states around the world; by 2050 the number will have climbed to three dozen. Some of the richest places on earth are shedding people every year: Japan, Korea, Spain, Italy, much of Eastern Europe. “We are a dying country,” Italy’s health minister, Beatrice Lorenzin, lamented in 2015.

But this isn’t the big news. The big news is that the largest developing nations are also about to grow smaller, as their own fertility rates come down. China will begin losing people in a few years. By the middle of this century, Brazil and Indonesia will follow suit. Even India, soon to become the most populous nation on earth, will see its numbers stabilize in about a generation and then start to decline. Fertility rates remain sky-high in sub-Saharan Africa and parts of the Middle East. Even here, though, things are changing as young women obtain access to education and birth control. Africa is likely to end its unchecked baby boom much sooner than the UN’s demographers think.

Some of the indications of an accelerating decline in fertility can be found in scholarly research and government reports; others can only be found by talking to people on the street. And so we did. To gather research for this book, we traveled to cities on six continents: to Brussels and Seoul, Nairobi and São Paulo, Mumbai and Beijing, Palm Springs and Canberra and Vienna. There were other stops as well. We talked to academics and public officials, but more important, we talked to young people: on university campuses and at research institutes and in favelas and slums. We wanted to know what they were thinking about the most important decision they will ever make: whether and when to have a baby.

Population decline isn’t a good thing or a bad thing. But it is a big thing. A child born today will reach middle age in a world in which conditions and expectations are very different from our own. She will find the planet more urban, with less crime, environmentally healthier but with many more old people. She won’t have trouble finding a job, but she may struggle to make ends meet, as taxes to pay for healthcare and pensions for all those seniors eat into her salary. There won’t be as many schools, because there won’t be as many children. 

But we won’t have to wait thirty or forty years to feel the impact of population decline. We’re feeling it today, in developed nations from Japan to Bulgaria that struggle to grow their economies even as the cohort of young workers and consumers diminishes, making it harder to provide social services or sell refrigerators. We see it in urbanizing Latin America and even Africa, where women are increasingly taking charge of their own destinies. We see it in every household where the children take longer to move out because they’re in no rush to settle down and haven’t the slightest intention of having a baby before they’re thirty. And we’re seeing it, tragically, in roiling Mediterranean seas, where refugees from wretched places press against the borders of a Europe that is already starting to empty out.

We may see it, very soon, influencing the global contest for power. Population decline will shape the nature of war and peace in the decades ahead, as some nations grapple with the fallout of their shrinking, aging societies while others remain able to sustain themselves. The defining geopolitical challenge in the coming decades could involve accommodating and containing an angry, frightened China as it confronts the consequences of its disastrous one-child policy.

Some of those who fear the fallout of a diminishing population advocate government policies to increase the number of children couples have. But the evidence suggests this is futile. The “low-fertility trap” ensures that, once having one of two children becomes the norm, it stays the norm. Couples no longer see having children as a duty they must perform to satisfy their obligation to their families or their god. Rather, they choose to raise a child as an act of personal fulfillment. And they are quickly fulfilled.

One solution to the challenge of a declining population is to import replacements. That’s why two Canadians wrote this book. For decades now, Canada has brought in more people, on a per capita basis, than any other major developed nation, with little of the ethnic tensions, ghettos, and fierce debate that other countries face. That’s because the country views immigration as an economic policy—under the merit-based points system, immigrants to Canada are typically better educated, on average, than the native-born—and because it embraces multiculturalism: the shared right to celebrate your native culture within the Canadian mosaic, which has produced a peaceful, prosperous, polyglot society, among the most fortunate on earth.

Not every country is able to accept waves of newcomers with Canada’s aplomb. Many Koreans, Swedes, and Chileans have a very strong sense of what it means to be Korean, Swedish, or Chilean. France insists its immigrants embrace the idea of being French, even as many of the old stock deny such a thing is possible, leaving immigrant communities isolated in their banlieues, separate and not equal. The population of the United Kingdom is projected to continue growing, to about 82 million at the end of the century, from 66 million today, but only if the British continue to welcome robust levels of immigration. As the Brexit referendum revealed, many Brits want to turn the English Channel into a moat. To combat depopulation, nations must embrace both immigration and multiculturalism. The first is hard. The second, for some, may prove impossible.

Among great powers, the coming population decline uniquely advantages the United States. For centuries, America has welcomed new arrivals, first from across the Atlantic, then the Pacific as well, and today from across the Rio Grande. Millions have happily plunged into the melting pot—America’s version of multiculturalism—enriching both its economy and culture. Immigration made the twentieth century the American century, and continued immigration will define the twenty-first as American as well.

Unless. The suspicious, nativist, America First groundswell of recent years threatens to choke off the immigration tap that made America great by walling up the border between the United States and everywhere else. Under President Donald Trump, the federal government not only cracked down on illegal immigrants, it reduced legal admissions for skilled workers, a suicidal policy for the U.S. economy. If this change is permanent, if Americans out of senseless fear reject their immigrant tradition, turning their backs on the world, then the United States too will decline, in numbers and power and influence and wealth. This is the choice that every American must make: to support an open, inclusive, welcoming society, or to shut the door and wither in isolation.

The human herd has been culled in the past by famine or plague. This time, we are culling ourselves; we are choosing to become fewer. Will our choice be permanent? The answer is: probably yes. Though governments have sometimes been able to increase the number of children couples are willing to have through generous child care payments and other supports, they have never managed to bring fertility back up to the replacement level of, on average, 2.1 children per woman needed to sustain a population. Besides, such programs are extremely expensive and tend to be cut back during economic downturns. And it is arguably unethical for a government to try to convince a couple to have a child that they would otherwise not have had.

As we settle into a world growing smaller, will we celebrate or mourn our diminishing numbers? Will we struggle to preserve growth, or accept with grace a world in which people both thrive and strive less? We don’t know. But it may be a poet who observes that, for the first time in the history of our race, humanity feels old.