Urban life can be trying—cars and buses honk, passersby jostle, concrete and brick win out over grass and trees. Researchers have known for decades that residents of densely populated areas have higher rates of mental illnesses, including anxiety disorders and schizophrenia. But do the brains of city dwellers function any differently from those of rural folk? Studies are showing that they do.

German researchers recently asked subjects from large cities, small cities and the countryside to undergo a standard psychological stress test—doing arithmetic under time pressure—while having their brain imaged with functional magnetic resonance imaging. Current city living, testing found, correlated with a boost in activity in a brain region called the amygdala, which is associated with memory and emotional intelligence, with a particularly large effect in people from big cities. Even more surprising, subjects who had grown up in a city showed higher activation of a brain area called the anterior cingulate cortex, essentially the amygdala’s boss, even if they had later moved to the suburbs or country. The findings were published this past summer in the journal Nature. (Scientific American is part of Nature Publishing Group.)

Both the magnitude and the specificity of the effect are surprising, says Andreas Meyer-Lindenberg, director of the Central Institute of Mental Health in Mannheim, Germany, and the study’s lead author. But he does not yet understand why these brain regions were more active in urbanites under stress. Another recent study suggests that the amygdala and anterior cingulate cortex become activated when one’s personal space is invaded. “Maybe it has to do with crowding,” Meyer-Lindenberg says.

The activation could reflect the neural machinery involved in managing human interactions, suggests Lisa Feldman Barrett, a psychologist at Northeastern University. She recently correlated amygdala volume with the size of a person’s social network. Does a larger or more strongly activated amygdala help you remember new people?

Knowledge of the underlying mechanism should help investigators answer this and other questions more quickly. Traditional epidemiology requires large numbers of subjects to identify broad effects, such as the link between urban life and mental illness. But now researchers can study smaller groups of subjects to see how specific factors—for example, noise in the home or proximity to a green space—play into mental illness and, more broadly, urban stress. Meyer-Lindenberg calls this newer field “neuroepidemiology.” That, in turn, could help city planners determine which design features would provide the most solace.