One In Four People Globally Live In A Big City, But Growth Is Slowing

The Megacity Boom Has a Natural Off-Ramp, Study Finds

Cities like Mumbai, Lagos, Sao Paulo, and Shanghai have spent decades swallowing up more and more of the world’s population, pulling in millions of people chasing jobs and opportunity. It has felt like an unstoppable trend, one where the biggest cities just keep getting bigger and bigger. A massive new study, built on nearly a century of population data, says that trend has a natural ceiling, and plenty of countries have already hit it.

Research published in the Proceedings of the National Academy of Sciences finds that big cities grow fastest early in a country’s development, but that edge fades away as more of the population moves into cities generally. Once a country becomes highly urbanized, that advantage all but disappears.

This matters far beyond academic curiosity. It shapes how governments plan roads, housing, and hospitals for decades to come. A future where megacities keep swallowing an ever-bigger share of humanity looks nothing like one where growth spreads out across cities of every size, and researchers say the difference adds up to hundreds of millions of people ending up in very different kinds of places by the end of the century.

A Data Problem That Stalled the Field

For years, comparing cities across countries was nearly impossible, because every country counted them differently. What counts as a “city” in Japan might not count in Nigeria, so studies kept reaching different, sometimes contradictory conclusions. Researchers solved that by building two new datasets from scratch. One covers 99 countries and 94% of the world’s population between 1975 and 2025, using satellite data to map where cities actually are instead of relying on shifting political boundaries. The other rebuilds the history of American cities from 1850 to 2020 using old census records, some handwritten and more than a century old, matching over 500 million individual records to nearly 40,000 places.

Big Cities Only Outgrow Small Ones Early On

In short, big cities do not automatically grow faster than small ones forever. It depends on where a country is in its development. In countries where most people still live outside cities, largely across Africa and Asia today, big cities are growing about 7.3% faster than the national average. In countries that have already urbanized, that edge is gone. In Europe, big cities grow at close to the national pace, and in the Americas, they have actually grown slightly slower than everywhere else.

South Korea and the United States each show this shift happening in real time across their own history. Big cities in both countries surged ahead early on, back when factories and jobs were concentrated in a handful of urban centers. As those countries urbanized further and opportunity spread to more places, that head start quietly faded, and city growth rates evened out.

Where Megacities Are Headed by 2100

Researchers ran the numbers forward to 2100 under four different scenarios to see where all this leads.

If city size stopped mattering entirely, 33% of the world would live in cities of a million-plus by 2100. If big cities kept their historical edge forever, that number jumps to 47%. Just extending current trends in a straight line lands at 42%.

Their own model, which factors in that big-city advantage fading as countries develop, lands at 38%, or about 450 million fewer people living in megacities than a straight-line guess would suggest. For context, just 11% of the world lived in million-plus cities back in 1975. That number had already more than doubled to 24% by 2025.

Researchers are upfront that 2100 is a long way off, and this is a projection based on past patterns, not a guaranteed outcome. They also cannot say for certain why the advantage fades. Slower migration from the countryside, rising housing costs, and the spread of jobs and business away from city centers could all be part of the answer.

Slower Growth Could Ease Pollution but Cost Jobs

That uncertainty matters for policy. Big cities tend to produce more per person and also strain the environment more per person, so a slowdown cuts both ways. Less megacity growth could mean fewer economic gains from concentrating people and business together, but it could also mean less pollution and fewer emissions concentrated in a handful of places. Which trade-off matters more is a judgment call for policymakers, not something the data alone can settle.

Wild cards like climate migration or a major geopolitical shift could throw off any of these projections. Still, the pattern behind this study has survived a lot already, including the rise of the automobile, the internet, and globalized trade, without breaking down.

Megacities are not done growing. But the free ride that once made them grow faster than everywhere else is already gone in much of the world, and the rest of the world looks to be catching up to that same finish line.

---