The link between psychosis and city living was first noticed by American psychiatrists in the early 1900s who found that asylum patients were more likely to come from built-up areas. This association was sporadically rediscovered throughout the following century until researchers verified the association from the 1990s onwards with systematic and statistically controlled studies that tested people in the community as well as in clinics.

One particularly extensive study using health records for almost the entire population of Denmark found that the risk of being diagnosed with schizophrenia increased in a small but proportional way as people spent more time spent living in urban environments. Many studies have since replicated this finding, with neighborhood levels of social deprivation seeming to amplify the association and levels of social integration seeming to reduce it.

To many, this provides evidence that cities are universally bad for our mental health—something that chimes with a strong cultural belief that associates the natural world with tranquillity. It might seem like common sense that living in a run-down, inner-city neighborhood would wear away at your psychological wellbeing. But here is where the cultural cliché breaks down, because the effect is surprisingly selective.

The data shows that urban environments reliably increase the chances of being diagnosed with schizophrenia or having related experiences like paranoia and hallucinations. This is not the case for other mental health problems primarily caused, for example, by depression or mood instability. If it was a general effect on wellbeing, you would expect the chance of being diagnosed with any mental health problem to increase at an equal rate, but this isn’t the case.

There are good reasons to think that city living might be the cause of some of these problems. The two big psychological negatives of city living, social isolation and social threat, are already well studied in mental health. They are risk factors for a range of psychological difficulties but have been particularly associated with misperceptions and paranoia. And for people who are already experiencing paranoid delusions, there is good evidence that urban environments amplify anxieties, increase the intensity of hallucinations, and weaken self-confidence.

But none of this conclusively proves that cities cause schizophrenia and some argue the causal arrow actually goes round the other way. People with psychosis, the alternative explanation goes, are just more likely to end up living in poor city neighborhoods—something first labeled the ‘social drift’ hypothesis.

Originally, this was a simple stereotype of someone hitting ‘rock bottom’ due to the disabling effects of psychosis. But we know this can’t explain what we see in modern studies because time spent in the city is also associated with your future chance of developing these experiences. A recent study led by Joanne Newbury at King’s College London even found this effect in children, who have little say where they live.