RESEARCHERS have long known that city dwellers are at greater risk of mental health problems than their country cousins. But the biology behind this was not known until now.

Results of an international study have shown for the first time how two regions of the brain responsible for regulating stress and emotion are affected by city living.

As part of the German study, 32 people were asked to do a tricky maths test while lying in a brain scanner.

Researchers at the University of Heidelberg introduced stress by imposing time constraints and relaying disapproving feedback from examiners.

They found that when exposed to stress the parts of the brain that process emotion - the amygdala and cingulate cortex - were more active among the students who lived in or had been raised in cities.

The findings, published in the journal Nature this week, establish that there is a connection between city living and sensitivity to social stress.

Results also showed a corelation between the activity levels in the amygdala region of the brain and the size of the city each student called home: people from cities of more than 100,000 people showed more activation of the amygdala region than those from towns of more than 10,000, and those in turn showed more activation than people from the rural areas.

Previous research has found that growing up in a big city raises the risk of schizophrenia.