The question is so obvious that you could easily forget to ask it: why do cities so often have a poor east side? To be clear, the mystery is not why every city has its leafy and its grubby sections – it costs money to live in nice places and to avoid nasty ones, which tends to group people into them by wealth. The mystery is why the poor groups always end up in the east.

Of course, the true picture is never neat nor simple, but by common consent a British-biased list of cities with poor eastern districts would include: London, Paris, New York, Toronto, Bristol, Manchester, Brighton and Hove, Oxford, Glasgow, Helsinki and Casablanca. No doubt there are some cities where poverty clusters in the west, but they seem harder to find; perhaps Delhi and Sydney?

The story seems easier to explain case by case. In London, for example, the docks are downstream in the east, and docks are rarely very salubrious in cities. For much of its history, the Thames also took the city’s waste, and smell, eastward. No wonder the poor wound up living there, you might say.

But then consider Paris, where the Seine flows westward, as does the Avon in Bristol. Or New York, which effectively has two rivers, and is a completely different shape. Or even Brighton, which has no rivers at all. Despite their differences, the story never varies: for poverty, look east.

One theory is that it’s all about air pollution. In the middle latitudes where most of the world’s cities can be found, the prevailing winds are westerlies, which means they blow to the east. Crudely, it has long been thought that they might take smoke and odours with them, and now a new study by Stephan Heblich, Alex Trew and Yanos Zylberberg for the Spatial Economics Research Centre suggests this theory might be right.

Detroit’s suburbs: in the US unlike the rest of the world, the outskirts tend to be home to the city’s more affluent residents. Photograph: Alamy

“This anecdotal discussion about pollution in the centre of cities and smoke drifting to the east is something that we have been documenting very precisely,” Zylberberg tells me. “Basically what we’ve been seeing in the past, because of pollution and wind patterns, is rich people escaping the eastern parts of town, because they were very polluted.”

For their research, published last November, Zylberberg and his colleagues built simulations of 70 British cities, including the sites of 5,000 industrial chimneys, as they would have been in 1880. Using mathematical models they claim to have been able to reconstruct the movement of air within the given topography and work out where the pollution would end up. They concluded that areas of high pollution were indeed more likely to become deprived areas, and found that they were generally in the east.

What we’ve been seeing in the past is rich people escaping the eastern parts of town, because they were very polluted Yanos Zylberberg

“Past pollution explains up to 20% of the observed neighbourhood segregation whether captured by the shares of blue collar workers and employees, house prices or official deprivation indices,” the paper says. It concluded that no British cities have wind patterns which ought to create a polluted west. Heblich, Trew and Zylberberg also looked for eastern poverty patterns before the industrialisation of the 19th century, and did not find them.

They also discovered that the deprivation of an area faded when the pollution did. “You can see this difference at the end of the 19th century,” says Zylberberg. “All neighbourhoods would converge back to the mean after that. The poor districts are not very poor. The rich districts are not very rich, and once pollution disappears, then everything comes back to normal.”

If pollution was severely concentrated to start with, however, segregation persisted even after it had gone, as the perception of schools, public amenities, infrastructure and reputation made the pattern hard to shift. Gentrification has begun to change this, but only in big, wealthy cities like Paris and London – and only recently.

The north-eastern Paris suburb of Bobigny, where poverty and unemployment are widespread. Photograph: Yoan Valat/EPA

The eastward drift is not the only pattern of deprivation. Big cities that grow rapidly often develop rings of poverty around a more affluent core, as poor people arrive from rural areas looking for work and have to live in the cheapest place they can find, which tends to be far away. At any rate, by the time these people arrive, it is the only place left to build new homes. Moscow and Paris, again, are good examples, as are most Chinese cities. Although, some of the new urban centres being built from scratch by the authorities may turn out differently.

One country is a famous exception to this rule. In the US, the suburban ring is generally a place of greater peace and affluence, while the inner cities are often associated with deprivation – Detroit being the most sadly famous case. Why things happened in reverse in the US is not quite clear, and is much discussed by academics. Some, such as Kenneth T Jackson, have argued that “a national distrust of urban life and communal living” plays an important role; but of course there are other factors.

By the end of the first world war, most US cities were established with industrial centres, but the success of the car in the interwar years made it possible for wealthy people to build larger homes for themselves on the outskirts, but within reach of work. Indeed, some of these new suburbs were specifically designed to exclude people.

“In 1916, the US supreme court, in Buchanan v Warley, ruled racial zoning was unconstitutional,” writes Ann Durkin Keating. “Nevertheless, many real estate developers utilised restrictive covenants to maintain racial exclusion … Racial restrictive covenants excluded certain groups of people (most often African Americans, but also Jews, Catholics and other groups depending on the locale) from ever owning or renting the property.”

In very general terms, therefore, you might say that people have ended up living in either new world or old world city patterns.

In the old world – mostly Europe and Asia – cities were already large and well established when industrialisation came. This meant the flood of poor workers had to go and live on the outskirts, and many of the factories went there, too, leaving the centre – especially its western section – looking old, clean and civilised.

New world cities, on the other hand, were often built around industrial centres. This made people wealthy, upon which they escaped to the suburbs by car – and defended them vigorously against intruders. Indeed, British cities with their leafy commuter belts and partially reclaimed centres are often quite American, by European standards. Perhaps we have our own strain of national distrust as well.

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