By 2050, two-thirds of the world's population will be living in cities. Although city living has many advantages, rapidly increasing urbanization has major health implications — schizophrenia is more common in people born in cities than in those from less heavily populated districts, and living in cities increases the rates of depression and anxiety disorders. It has been suggested that social stress plays a part in these effects, but the mechanisms involved are unknown. Now, in a study of healthy German volunteers using functional magnetic resonance imaging, a key brain structure for negative emotion (the amygdala) was found to be more active during stress in city dwellers, and a regulatory brain area (the cingulate cortex) more active in people born in cities. These results identify potential mechanisms linking social environment and mental illness, and might contribute to planning healthier urban surroundings. Cover image: O. Dusegård/Getty.