L OOK UP, AND cities seem to be squeezing in more people. All of the world’s 73 residential towers over 250 metres high were built after the year 2000. Another 64 are under construction. On 57th Street in New York, a building where The Economist used to have an office has been knocked down and replaced by an 82-storey glass splinter. When finished, it will be just 8 metres shorter than the Empire State Building.

But appearances can deceive. Shlomo Angel and researchers at the Urban Expansion Programme at New York University have used population data and satellite maps to show that most cities are becoming less densely populated. That is seldom because they are losing people (although New York is). Usually, it is because they grow faster in extent than in population. In 1990-2014, for example, Mexico City grew from 9.8m inhabitants to 17.8m, an 82% gain. During the same period, however, its built-up area expanded by 128%. This pattern is common. Sprawl has outpaced densification in 155 of the 200 cities tracked by the Urban Expansion Programme.

As people grow richer, they demand more space. Despite the efforts of many urban planners to stop them, they move from cramped inner cities to sparsely populated fringes (Mr Angel’s team counts suburbs as parts of cities, regardless of where political boundaries lie). Moreover, because people are living longer and having fewer children, a growing proportion of households contain only one or two people.

Even the towers that spring up in city centres are not all that dense. There is a lot of air between them and a lot of elevator shafts inside them. High-rise cities like Seoul and Tokyo are less densely populated than Dhaka, the capital of Bangladesh, where most people live in walk-up apartment buildings or low-rise slums.

Cities can be dense in different ways. Hong Kong is a champion at stacking people on top of each other. But almost all of Hong Kong’s built-up area is occupied by roads, pavements, offices, hotels, parks and mandatory spaces between buildings. The footprints of residential buildings account for less than 4% of it. In Dhaka, by contrast, homes cover nearly 20% of the land. In a poor city like Kinshasa, the capital of the Democratic Republic of Congo, population density comes mostly from squeezing more people into each room.

Many low-density cities wish to change. Minneapolis, for example, plans to alter housing codes to pack more people in. But density always comes with drawbacks. Towers cast shadows. Devoting more of the city to residential buildings means less space for other useful things—skimp on roads and you might end up with Dhaka’s traffic jams. And nobody should envy the residents of Kinshasa. It is always worth asking the advocates of higher density: what kind, exactly, would you like? ■

Sources: Atlas of Urban Expansion; “Anatomy of density I: six measurable factors that together constitute urban density (forthcoming)” by Shlomo Angel and Patrick Lamson-Hall (2019)