Cities can be stressful places, and are a far cry from the sparsely populated landscapes in which our prehistoric ancestors evolved. All of that noise, traffic, pollution and crowding has a well-documented impact on our mental health.

People who live in cities are more likely to have mood or anxiety disorder (21 percent and 39 percent, respectively) and are twice as likely to have schizophrenia. With more than half of the world's population currently living in urban areas—and about 70 percent projected to be city dwellers by 2050—figuring out how to curb the mental toll of city life could become a major public policy issue. And Jens Pruessner, of the Douglas Mental Health University Institute at McGill University, calls it "a cause for concern."

But the biological bases for these urban and rural differences have been unclear. New research shows that living—and/or being raised—in urban environments impacts specific regions of the brain that deal with stress responses.

Researchers observed the brains of 32 healthy German adults via an fMRI machine as the volunteers underwent social stress tests. During the tests, subjects had to answer tough math questions (with a success rate hovering around 25 to 40 percent) under time pressure—all while getting negative verbal responses.

Pruessner and his colleagues found that two areas of the brain looked substantially different in the urban dwellers. Current urbanites had higher activation in their amygdala, which is linked to response to threats. And those who had lived in cities during the first 15 years of their life showed changes in the pregenual anterior cingulate cortex, which is charged with moderating amygdala and other stress regulation. For the study, cities were defined as municipalities with more than 100,000 people and towns as areas with a population of greater than 10,000 (to contrast with more sparsely populated rural areas).

Two similar subsequent experiments have arrived at similar results, providing support for the initial findings. A paper outlining all of the new work published in the June 23 issue of Nature (Scientific American is part of Nature Publishing Group).

The study itself shows only the correlation between city living and the shifts in stress processing—rather than cities as a source of the diseases themselves. But, as the researchers noted in their paper, the amygdala "has been strongly implicated in anxiety disorders, depression and other behaviors that are increased in cities, such as violence," and the pregenual anterior cingulate cortex "is implicated in processing chronic social stressors such as social defeat." The team also found that subjects who had been born and raised in cities had weaker connections between these two regions, a condition which "has previously been associated with genetic risk for psychiatric disorders," Daniel Kennedy and Ralph Adolphs, both of the California Institute of Technology, noted in an essay about the new work, published in the same issue of Nature.

Even though city dwellers showed differences in their brains, their levels of the stress hormone cortisol was on par with their rural peers. Kennedy and Adolphs also noted that city populations might, to a certain extent, be a self-selecting population. "There are wide variations in individuals' preferences for, and ability to cope with, city life: some thrive in New York City; others would happily swap it for a desert island," they wrote in their essay. One key reason for this might be "the perceived degree of control that people have over their daily lives."

But as the researchers noted, even the apparently easily stressed urbanites in the study "grew up in relative safety and prosperity in Germany," whereas billions of other people are living under less affluence among greater disparities, which could lead to even more drastic changes in the brain.

And the global move to urban environments doesn't show any signs of abating, so keeping city folk mentally healthy will likely have to be a focus going forward. Kennedy and Adolphs suggested future work focusing on ways of "softening" the urban landscape via better architecture and urban planning.

But before more psychologically healthful cities can be designed and built, more work needs to be done to establish the mechanisms behind these changes in the brain. Previous research suggests that the impact of an urban environment might start as early as the womb, via mothers under stress. "Determining the biology behind this is the first step to remedy the trend," Pruessner said in a prepared statement. And that new knowledge should "point to a new approach to interface social sciences, neurosciences and public policy to respond to the major health challenge of urbanization."

Image courtesy of iStockphoto/Razvan