Most leading projections show that the world’s population will peak in the second half of the twenty-first century. But what happens after that?

Long-term projections by researchers at the University of Texas at Austin find that today’s large global population may be a blip in human history, and that fertility trends could have compounding consequences for population size and age structure far into the future.1 If fertility stays low, four-fifths of all human births may already have occurred.

Dean Spears, Sangita Vyas, Gage Weston, and Michael Geruso from the University of Texas at Austin project how the world’s population could change well beyond the year 2100 under two sets of scenarios1: one where fertility stays low, and another where it eventually rebounds. Without a recovery, they state, “the 400-year span when more than 2 billion people were alive would be a brief spike in history.”

Their work comes into a world already well into a fertility transition. Two-thirds of people now lives in countries with fertility rates below replacement level (the point at which each generation is just large enough to succeed the last, roughly 2.1 births per woman), and countries that fall below replacement tend to stay there. Yet projections that extend these trends across centuries are rare.

It’s worth noting that these are illustrative scenarios, not forecasts. No demographer expects fertility, mortality, or migration to stay frozen indefinitely. As Spears puts it, the projections are “conditional statements” about what the world would look like if it followed one of these stylized paths. The real value is in the comparison: Even small differences in fertility levels and trends, sustained over generations, can lead to vastly different futures.

The Farther Fertility Falls, the Faster Population Declines

In their first set of scenarios, the authors assume that after 2100, global fertility converges to different below-replacement levels, which are already common in many parts of the world. In all cases, the global population eventually declines, although the timing and speed depend on just how far fertility falls.

The differences are stark. At 1.2 births per woman—roughly East Asia’s current rate—the global population would drop below 1 billion by about 2340 and reach 500 million by 2380 (see figure). At 1.5—similar to present-day Europe—those same thresholds would be delayed by roughly 25 and 95 years, respectively. If global fertility converges to 1.8 births per woman—about the level today in Mexico—the population would remain above 1 billion until after the year 2700.

On a fundamental level, population changes tend to lag behind fertility changes because age structure continues to shape growth—what demographers call “population momentum.” But the relationship also compounds, and each generation’s size influences the number of births in the next. Just as above-replacement fertility drove the rapid growth of the last two centuries, sustained low fertility could run that process in reverse.

With a Fertility Rebound, It’s All About Timing

In a second set of scenarios, the authors ask: If fertility rebounds to replacement level, how much does the timing matter? The answer is a great deal.

Even if fertility recovers and the population stabilizes, the eventual size depends on how long fertility stayed low. A rebound around 2125 would leave a stabilized population of roughly 8 billion. Delay that rebound to 2175, and the long-run population could be just 2 to 4 billion, depending on how low fertility initially fell.

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