Now researchers have found a possible reason why. Imaging scans show that in city dwellers or people who grew up in urban areas, certain areas of the brain react more vigorously to stress. That may help explain how city life can boost the risks of schizophrenia and other mental disorders, researchers said.
Previous research has found that growing up in a big city raises the risk of schizophrenia. And there's some evidence that city dwellers are at heightened risk for mood and anxiety disorders, although the evidence is mixed.
In any case, the volunteers scanned in the new study were healthy, and experts said that while the city-rural differences in brain activity were intriguing, the results fall short of establishing a firm tie to mental illness.
The study, done in Germany and published in Thursday's issue of the journal Nature, focused on how the brain reacts to stress caused by other people.
To do that, investigators had volunteers lie in a brain scanner and solve math problems. The volunteers expected easy problems, but they were in fact hard enough that each volunteer ended up getting most of them wrong.
While in the scanner, volunteers heard a researcher criticize their poor performance, saying it was surprisingly bad and disappointing, and telling the volunteers they might not be skilled enough to participate.
An initial study with 32 volunteers found city-urban differences in two brain areas. One was the amygdala, which reacts to threats in one's environment, and the other was circuitry that regulates the amygdala. Researchers found that volunteers from cities of more than 100,000 showed more activation of the amygdala than participants from towns of more than 10,000, and those in turn showed more activation than people from rural areas.
To assess any effect of where the volunteers grew up, the researchers assigned each an "urbanicity" score based on how many years they'd spent by age 15 in a city, town or rural area. The higher the score, the more urban their childhood life was, and the more activity showed up in the amygdala-regulating circuitry during the experiment.
A slightly different stress-producing test produced similar results with a different group of 23 volunteers.
But when a third group of 37 adults did mental tasks without being criticized for poor performance, they showed no urban-rural differences. That shows the effect comes from the criticism rather than just doing the mental task, the researchers said.
The study can't reveal why city life would boost the brain responses, but it could be because of the stress from dealing with other people, said Dr. Andreas Meyer-Lindenberg, director of the Central Institute of Mental Health in Mannheim, Germany, and senior author of the report. Animal studies suggest that early exposure to stress can cause lasting effects, he said.
Jens Pruessner, a study co-author from the Douglas Mental Health University Institute in Montreal, said the study illustrates a new avenue for understanding the risk factors for developing mental illness.
An expert in emotion and the brain who wasn't involved with the study, Elizabeth Phelps of New York University, said it's premature to draw conclusions about what the results mean for mental illness.
"These results are interesting but preliminary," she said. "This will raise a lot of interest in this idea. Whether or not it pans out in future research, who knows, but I think it's worth investigating."