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Humans have always tried to glimpse the future. Our methods have improved — from the stars and tarot cards to complex election forecasts. People love predictions, making them and reading about them. How else to explain the popularity of palm readers and legal gambling markets for sports, politics, and more?

There’s one kind of prediction that does not capture the public’s imagination as often but is nevertheless instrumental in shaping how politicians and other powerful people contemplate humanity’s future. Welcome to the world of population projections.

Estimates from highly respected demographers suggest that the United States of the future might be less populous than that of the past. Those predictions have galvanized the political right, combining a sense of cultural decline with xenophobia to persuade voters that a return to traditional American values — which means more child-rearing — is the solution.

Until recently, the fear had not been too few humans, but too many. A few generations ago, a widespread panic was set off by the dire projections published in the infamous book. The Population Bomb, which warned of a wave of famines in the developing world because there would be too many people to feed.

National leaders are now confronting a question that would have been unthinkable a century ago: What if we don’t have enough people?

This dramatic reversal reflects the inherent uncertainties with population projections. These statistical estimates of the future of our species are imperfect but they are necessary. There are small groups of demographers, scattered around the world at some of our most revered institutions, asking some of the biggest questions of all: How many people will be alive 50 years from now? In 100 or, even 200?

These estimates have evolved from the days when overpopulation fears were rampant. Whereas studies once appeared to show such robust population growth that people feared mass starvation, they now anticipate stagnating or declining populations around the world. Already, more than half of the world’s nations are below the replacement fertility rate of 2.1 children per woman. The University of Washington’s Institute for Health Metrics and Evaluation projects that by 2100, nearly every country will have a fertility rate lower than what would be necessary to sustain the human population over time.

But the farther in the future you go, the more uncertain population predictions get — those that project centuries from now are little more than a guess. The United Nations’ current spread of projections for the global population in 2100 goes from less than 7 billion people on the low end all the way to more than 14 billion — representing two completely different futures for our species. The median guess is roughly 10 billion.

These sketches of humanity’s future are not carbon copies of each other, either. Population projections from the UN, the US Census Bureau, the University of Washington’s Institute for Health Metrics and Evaluation (IHME), and the International Institute for Applied Systems Analysis (IIASA), to name a few of the most prominent groups, often share a general trajectory but can significantly differ in specifics. When added up over the decades, the divergences are sometimes stark.

Everyone is reaching for answers because, as any good demographer might tell you, we have never seen a country rebound from low fertility rates. The trends have been down, down, down. Japan and South Korea, at the leading edge of this problem, are now in a genuine demographic crisis — the kind that awaits the US and Europe if the projections are to be believed, the kind that is already spurring so much societal angst.

We’re all headed toward a smaller world. … The entire global economy has to adjust to an unprecedented reality,” Jennifer Sciubba, president and CEO of the Population Reference Bureau, told me. As she explained in a 2023 interview with WBUR: “All of our theories about the good life, our economic theories, our political theories — those were all developed under conditions of population growth and economic growth. … It’s really hard to get a paradigmatic shift and say, ‘What if we try to look at the world in a different way?’”

In other words, these projections aren’t just academic data computations. They matter. So it’s worth understanding how they work.