From 2000 to 2020, 10,727 cities added 785 million people. Migration explains 354 million. Births explain 432 million.
A new studies in Nature Cities find that Urban growth is actually actually several different demographic engines running at once.
The researchers used annual WorldPop age and sex population grids, matched them to consistent global city boundaries, and reconstructed how more than 10,000 cities changed over two decades.
That lets them look inside the national average.
A country can look young overall while its largest city is full of working-age migrants. Another city in the same country can be dominated by children. A third can be ageing. Once those differences are averaged together, the planning problem disappears from view.
The first big finding is that cities became more working-age.
Globally, the ratio of children and older people to working-age adults fell from 0.87 in 2000 to 0.59 in 2020. In simple terms, there were fewer dependants for every working-age adult.
That sounds like a demographic dividend. More workers, fewer dependants, more economic potential. But city-level data shows how uneven that dividend is.
Nigeria is a pretty good example. In 2020, Kano had a young dependency ratio of 0.83. Lagos was 0.47. It's the same country, but it has very different urban age structures.
Kano has a much larger share of children relative to working-age adults. Lagos has a much larger working-age population. One city faces heavier pressure on schools, childcare, vaccination and basic services. The other has more of the age structure that can support near-term economic growth.
Ethiopia shows an even sharper contrast. Bore had the highest urban dependency ratio in the dataset in 2020, at 1.26, while Addis Ababa was 0.34.
The second finding is that city size matters.
Smaller cities, especially in Africa, are much younger than larger cities. In African cities below 50,000 people, the median young dependency ratio approaches 0.9. In cities above 300,000, it is closer to 0.4.
That is a large gap in who cities are actually built for.
The explanation is intuitive. Smaller cities are often closer to rural economies, where fertility remains higher. Larger cities pull in working-age adults looking for jobs. The result is a kind of demographic sorting.
Workers concentrate in larger cities, and children make up a bigger share of smaller cities.
That creates a hard planning problem. Smaller cities often have less money, weaker infrastructure and thinner institutions. They may also have larger future needs, because they have more children to educate, transport, vaccinate and protect from heat, flooding and food shocks.
The third finding is about sex ratios.
Across the dataset, cities are male-dominated on average, with around 120 males for every 100 females. But again, the global average hides the real pattern.
In parts of the Middle East and North Africa, some working-age sex ratios exceed 2. That means more than two working-age men for every working-age woman.
That is the demographic signature of labour migration.
Men move for construction, logistics, infrastructure and urban service work. Some cities absorb that workforce at huge scale. The result is an urban population that looks very different from the country around it.
That affects housing demand. It affects labour markets. It affects heat exposure. It affects occupational safety. It affects who is actually present in a city when a crisis hits.
An interesting part of the study is that it basically gives urbanisation a balance sheet.
• A city growing through migration needs housing, jobs, transport links and labour protections.
• A city growing through natural increase needs schools, clinics, childcare, vaccination systems and long-term infrastructure for a young population.
• A city ageing into higher dependency needs healthcare, accessible transport, heat protection and social support.
• A city with a large male migrant workforce needs worker housing, occupational safety, heat rules and services for people who may sit outside formal systems.
So planning is basically not just whether a city is growing, but what is actually driving the growth.