Life in the city can be taxing. City dwellers often face higher rates of crime, pollution, social isolation and other environmental stressors than those living in rural areas. For years studies have consistently linked the risk of developing schizophrenia to urban environments—but researchers are only beginning to understand why this association exists. Addressing the link is increasingly urgent: According to a recent U.N. report, the proportion of people living in cities will rise from 54 percent of the world’s population in 2014 to 66 percent by 2050.

Researchers first suggested in the 1930s that urban living might increase schizophrenia risk. Since then many large epidemiological studies have reported an association between the two, primarily in European countries such as Sweden and Denmark. Converging evidence has revealed that growing up in the city doubles the risk of developing psychosis later in life. Studies have also begun to find that urban environments may heighten the risk of other mental health issues such as depression and anxiety.

A number of factors, including elements of the social environment (such as inequality and isolation) and physical stressors (such as pollution and noise) could explain how the city erodes well-being. Conversely, people predisposed to mental illness may simply be more likely to move into urban environments. Two studies published this month shed new light on these effects and suggest both scenarios could be involved.

Growing up in the urban jungle

Although the majority of investigations have focused on adults, studies suggest that exposure to urban environments early in life—being born or growing up in a city—matters most. To look more closely at this critical stage of life, a group of researchers led by Helen Fisher, a psychologist at King’s College London, and Candice Odgers, a psychologist at Duke University, conducted a longitudinal study involving 2,232 twin children in the United Kingdom.

The researchers used neighborhood surveys to determine whether twins lived in urban or rural environments at ages five and, later, 12. (Approximately half the children lived in cities at both time points). To further assess the characteristics of these neighborhoods, they used geodemographic data, interviewed mothers and surveyed neighbors. Finally they measured psychotic symptoms by conducting in-depth interviews with the children at age 12 to determine whether they had experienced hallucinations or delusions.

Their analysis revealed that growing up in the city nearly doubled the likelihood of psychotic symptoms at age 12, and that exposure to crime along with low social cohesion (that is, a lack of closeness and supportiveness between neighbors) were the biggest risk factors. Although most kids who have psychotic symptoms will not develop schizophrenia as adults, Fisher notes, “In some of the other studies where we follow people later in life, we show that [psychotic symptoms] are actually related to lots of other [mental health] problems as well, so it's a broader marker for that.” These problems include depression, post-traumatic stress disorder and substance abuse.

“This [study] adds to our own experimental evidence that strongly leads us to suspect that being in the city does something to a specific circuit in the brain that impairs your ability to deal with social stress,” says Andreas Meyer-Lindenberg, director of University of Heidelberg’s Central Institute for Mental Health in Germany. Meyer-Lindenberg’s group previously found that people who were living in or grew up in cities showed stronger activation in the amygdala and cingulate cortex (brain areas involved in processing and regulating emotion), respectively, compared with those from rural areas. More recently, they discovered that migration, another well-established risk factor for schizophrenia, led to similar alterations in brain function.

Who lives in cities?

Epidemiological studies provide strong evidence that an urban upbringing could contribute to poor mental health. Yet schizophrenia is a highly heritable disorder, meaning genetic factors may also contribute. One process that might be occurring is social drift, whereby people with mental illness tend to move into poor, deprived city neighborhoods. In a recent study, published this month in Translational Psychiatry, a group led by researchers at the University of Oxford assessed genetic and environmental influences in three different cohorts of Swedish individuals: 2,386,008 siblings, 1,355 twin pairs and molecular genetic data collected from blood samples in another group of twins. Their analyses revealed that the link between schizophrenia and the chances of living in a deprived neighborhood later in life was itself influenced by genetic factors.

The authors see genetics as a stronger explanation than urban living for explaining the occurrence of mental illness. “The key issue we're trying to address here is selection, who ends up living in deprived neighborhoods and why,” says Amir Sariaslan, a postdoctoral researcher in psychiatry at the University of Oxford. “You can't assume without testing for this, that [environmental effects] are causal.”

He believes prior studies may have overstated the importance of city-related environmental influence on schizophrenia. “I have not seen a single study that has adequately addressed familial confounding in the association between urban living and similar sort of exposures and later adverse outcomes,” Sariaslan says. Many epidemiological studies assess familial risk by accounting for family history, but another study conducted by Sariaslan and his colleagues (published 2015 in Schizophrenia Bulletin) found that this had a much smaller effect than cousin and sibling comparisons.
Most researchers agree that the specific factors associated with city living causing the increased risk for psychosis have yet to be ascertained, but not all share Sariaslan’s conclusion. “This study does not compare, in my view, with the very, very strong evidence that does suggest an environmental effect of being born in a city,” Meyer-Lindenberg says. One of his concerns with the current study is that it focused on residence in adulthood, when it is very likely that the effects of the urban environment happen around birth or early childhood. In fact, another recent study that found evidence for social drift concluded that this effect still could not explain the mental health risk in urban areas and pointed to the importance of considering whether a study addressed risk before or after disease onset.

Scientists will likely need to combine the hereditary and environmental factors to understand how city life impacts mental health. “Emphasizing the role of genes over the environment—or vice versa—is an overly-reductionist approach to the science, and ignores the fact that both sets of factors are relevant to psychosis onset,” says James Kirkbride, a psychiatric epidemiologist at University College London who was not involved in the new studies. “No one is denying genetic factors, overall, contribute a greater extent to risk, but of the two, only environmental influences can be ameliorated currently.” According to Kirkbride, the science confirms that efforts to reduce the negative impact of urban living should focus on disadvantaged neighborhoods, where the cycle of poor mental health may persist across generations.