NEW YORK – This may come as no surprise to residents of New York City and other big urban centers: Living there can be bad for your mental health.

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.

The study, done in Germany and published in today’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.

A slightly different stress-producing test produced similar results with a different group of 23 volunteers.

The study didn’t show 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, senior author of the report.