On November 15—as estimated by demographers—the count of humans on this planet reached eight billion. Population growth has been steady over the past few decades, with billion-person marks coming every dozen years or so. But that pattern is changing. Growth is beginning to slow, and experts predict the world’s population will top out sometime in the 2080s at about 10.4 billion.

Population curve shows a steady increase since 1960, projected to top out at 10.4 billion before 2100.
Credit: Katie Peek; Source: World Population Prospects 2022, United Nations Population Division

That slowdown is partly the result of a shift toward fewer offspring—a phenomenon that is happening almost everywhere but at different rates. High-income nations currently have the lowest birth rates, and the reverse is true: nations with the highest birth rates tend to have the lowest incomes. “The gap has continued to widen between wealthy nations and poorer ones,” says Jennifer Sciubba, a social scientist at the Wilson Center in Washington, D.C., who has written about these planetary-scale demographic shifts.* “But longer term, we’re moving toward convergence.” In other words, the current disparity among nations’ birth rates isn’t a permanent chasm. It’s a temporary divide, which will narrow over the coming decades.

Many factors contribute to the waxing and waning of the world’s population, such as migration, mortality, longevity and other major demographic metrics. Focusing on fertility, however, helpfully illuminates why the total number of humans on Earth seems set to fall. Demographers define fertility as the average total number of live births per female individual in a region or country. (In the accompanying graphics, the term “woman” is used to encompass anyone assigned female at birth.) The U.S.’s current fertility rate, for example, is about 1.7; China’s is 1.2. Demographers consider a fertility of 2.1 to be the replacement rate—that is, the required number of offspring, on average, for a population to hold steady. Today birth rates in the wealthiest countries have dropped below the replacement rate. Over the coming decades, most of the rest of the world’s countries will likely follow suit. Here’s how that might look.

In 1960, when the world’s population was three billion, nearly every country had fertility rates above 2.1 live births per woman.

Chart shows fertility rates for 217 regions in 1960, color-coded by income. All but 5 exceed the replacement rate of 2.1.
Credit: Katie Peek; Source: World Population Prospects 2022, United Nations Population Division

But over the subsequent decades, that began to change. A country’s fertility rate tends to be correlated to its average income. Wealthier countries were the first to move toward fewer offspring, but lower-income countries are also following the same trend.

Charts break down falling fertility rates from 1974 to 2010. By 2010, most high- and high-mid-income regions were below 2.1.
Credit: Katie Peek; Source: World Population Prospects 2022, United Nations Population Division

Here’s the picture today, as we crest eight billion.

By 2022, birthrates in Hong Kong and South Korea dipped below one baby per woman. The U.S. rate is currently 1.66.
Credit: Katie Peek; Source: World Population Prospects 2022, United Nations Population Division

Fertility was especially different in higher- and lower-income countries from the 1990s through today. But by the end of the century, fertility rates worldwide will reconverge at a lower number.

Charts show projected fertility rates. By 2058, about one quarter of regions are projected to have above-replacement levels.
Credit: Katie Peek; Source: World Population Prospects 2022, United Nations Population Division

These numbers provide insight into how—and where—the population growth rate is changing. But humanity’s future clearly depends on many things besides fertility. For example, people in wealthier nations may produce fewer children, but those offspring tend to consume more resources—so rich countries can still have outsize planetary impacts despite their dwindling populations. Organizations such as the United Nations Department of Economic and Social Affairs—which tracks and predicts human population numbers—are working toward policy-based solutions for how all of us can have healthy, satisfying and sustainable lives on Earth. A clear-eyed understanding of population shifts is critical for reaching that bright future.

*Editor’s Note (12/12/22): This sentence was edited after posting to correct Jennifer Sciubba’s current affiliation.