The claim:

Living in a city makes people develop schizophrenia.

Tell me more:

The claim is not quite that stark, but it’s close. For a study published last week, researchers interviewed 2,063 British twins (some identical, some not) at age 18 about “psychotic experiences” they’d had since age 12—such as feeling paranoid, hearing voices, worrying their food might be poisoned, and having “unusual or frightening” thoughts. Among those who lived in the most densely populated large cities, 34 percent reported such experiences; 24 percent of adolescents in rural areas did.

The twins are part of a long-running study that has followed them from birth in 1994-95, so the researchers— led by Helen Fisher of King’s College London and Candice Odgers of Duke University—also knew the teens’ family income, parents’ education, where they lived, and more. Conclusion: 18-year-olds raised in big cities were 67 percent more likely to have had psychotic experiences, the researchers reported in Schizophrenia Bulletin.

They then used standard statistics tools to account for possible psychosis-related factors other than cities per se. Cities have more people who are poor and uneducated, which are risk factors for schizophrenia and other forms of psychosis, so they controlled for socioeconomic status. Family psychiatric history raises the risk of an individual’s developing psychosis, and since there is some evidence that people with mental illness move to cities, which have more treatment facilities, the researchers controlled for this, too. They also controlled for drug use, some forms of which are more common in urban than rural areas. These calculations brought the extra risk of psychosis among urban teens down to 43 percent.

Really?

This study is far from the first to link city living to psychosis. In 2005, psychiatrists wrote that a higher prevalence of psychosis in cities “is one of the most consistent findings in schizophrenia research.” A Swedish study and a Danish one even found that the longer someone lives in a city, the greater the risk of schizophrenia—the sort of “dose response” that makes causality more likely.

But these results can reflect factors other than “cities make you crazy.” For instance, some people who develop mental illness move from small towns to cities for treatment; so do some families whose teens are showing early signs of psychosis. That skews the numbers from rural areas to urban ones without the latter actually causing the mental illness. A 2016 study of 1.5 million siblings, led by Amir Sariaslan of Sweden’s Karolinska Institute, even found that genetic variants that increase the risk of schizophrenia also make it more likely that someone will live in a deprived neighborhood—the kind that’s more common in a city than a rural area. He called the new study “potentially interesting,” but said it “is pretty much the same type we see over and over”: researchers try to control for confounding variables (socioeconomic status, family psychiatric history, and others), but confounders that researchers never suspect, especially genetic ones, lead to misleading conclusions.

“There are genetic risk factors that simultaneously explain why certain individuals have a higher propensity to live in deprived neighborhoods and to [have] psychotic experiences,” he said, something he and colleagues showed in a 2014 study.

Another factor that increases the rate of schizophrenia in cities: Immigrants have historically clustered in cities more than rural areas, explained Dr. Jeffrey Lieberman, of Columbia University, an expert in psychosis who was not involved in the new study. They also have historically had higher rates of psychosis, partly because not speaking the local language or understanding local customs can cause the kind of stress that tips a vulnerable person into schizophrenia.

The biggest reason to question whether, or how much, city living increases the risk of developing schizophrenia comes from the study itself. After the researchers teased out features of cities such as greater social isolation and “neighborhood disorder” (litter, vandalism, street crime), living in a city meant a 35 percent higher risk of psychotic experiences—which, their calculations showed, could have been due to random chance. In scientific parlance, the 35 percent was not statistically significant. Fisher argued that statistical significance “is not really the key thing here,” but as the paper cautioned, “causality … from this observational study cannot be assumed.”

Cities are defined by population density and size. Other attributes—social isolation, rundown buildings, street crime—are not eternal verities. If cities do cause some people to develop schizophrenia, rather than draw people who are at risk for that, these variables are likely responsible. “To the extent that urban environments become more stable and socially supportive,” therefore, “it could have a mitigating effect” on any increased risk for schizophrenia from living in a city, said Lieberman.

The verdict:

Some aspects of some cities at some times might tip some vulnerable people into schizophrenia, but the “urbanicity” effect has explanations other than “cities make people crazy.”

Republished with permission from STAT. This article originally appeared on May 30, 2017