City dwellers more sensitive to social stress than country folk

Urban life has some definite benefits over rural life—more job opportunities, better sanitation, and being able to get good take-out at any time of the day or night. However, living in a city also increases the odds that you suffer from anxiety, schizophrenia, and other mental disorders. While this relationship has been known for quite some time, researchers haven’t pinpointed what it is about city living that puts people at risk for these disorders. Now, a study in Nature comes a step closer to understanding this association, reporting that the brains of city dwellers are particularly sensitive to social stress.

Because of the daily demands of city life, the researchers hypothesized that living in an urban area may change how we respond to a particular kind of stress, called social evaluative stress. Social evaluative stress includes all the trials and tribulations of interacting with other people, such as the fear of making mistakes in public and concern about what others think of you.

To get a picture of social evaluative stress, German volunteers were asked about their current living situation and the type of area in which they were brought up. The researchers defined a city as having over 100,000 inhabitants, a town as having more than 10,000 people, and a rural area as having fewer than 10,000. The participants were then asked to perform a series of tests in an fMRI machine.

The tests, called the Montreal Imaging Stress Test (MIST) are meant to elicit social stress; they involve solving a series of mathematic questions in a limited amount of time. The participants could see how well they were doing, but they were not aware that the difficulty of the questions was manipulated so that they never surpassed a success rate of 40 percent. To elicit social evaluative stress, the researchers provided negative feedback after every few questions, telling the volunteer that it was important to do well on this test, and that they weren’t performing up to par. (Yes, it all seems rather cruel.)

By monitoring heart rates, breathing rates, and hormonal levels, the researchers knew they had successfully stressed the participants out. The fMRI images showed that people who were currently living in cities experienced higher amygdala activity than those who lived in towns; those who lived in rural areas experienced the lowest activity levels in the amygdala. Urban upbringing had a completely different effect: participants that grew up in cities experienced heightened activity in the periguenal ACC.

To determine whether the result was due specifically to social stress, or just to being tested in general, the scientists repeated the test. This time, instead of using the MIST, simple memory tests were administered, with no feedback at all. Here, there were no obvious correlations between urban living and any areas of heightened brain activity.

The amygdala and the ACC are part of a neural circuit that regulates our negative emotions and determines how we respond to threats. These regions are also known to be involved in schizophrenia and mental disorders involving anxiety and violence.

The scientists behind this work aren’t sure whether some environmental risk factor, such as pollution, toxins, or noise, plays a role in the association between city life and increased activity in these regions, or whether simply experiencing repeated instances of social stress might be responsible for the outcome. In any case, it’s a promising step toward understanding how where we live influences who we are, what we feel, and how we behave.

Nature, 2011. DOI: 10.1038/nature10190 (About DOIs).