Full of blaring horns, flashy billboards and fast-talking residents, cities can be stressful places to be.
So, it may be no wonder that urban-dwellers suffer from higher rates of schizophrenia, depression and other mental health problems, compared to people who live in quieter places.
Now, a new study helps explain why cities stress us out, and the answer seems to lie in our brains.
In stressful social situations, the study found, people who live in cities showed extra activity in a brain area that is associated with depression, anxiety and even violence. People who live in rural areas but were born in cities showed a similar kind of brain hyperactivity, the study found, suggesting that your place of birth may forever affect the way you deal with stress.
The researchers don't recommend that urbanites run out to sell their condos in favor of country homes. After all, there are plenty of reasons why living in a city can be good for both mental and physical health, including a plethora of stimulating environments and easier access to good health care.
Instead, the findings offer researchers and urban planners a new way to figure out what it is about cities that induce anxiety. And that, in turn, could help in designing cities that can reduce -- instead of invoke -- stress.
"We're not like Jane Austen, saying that cities are bad and countries are great," said lead researcher Andreas Meyer-Lindenberg, a psychiatrist at the University of Heidelberg's Central Institute of Mental Health in Mannheim, Germany. "It's not black and white."
"The thing is that it was known before that cities are a risk factor for mental health," he added. "What we show is how and what underlies that in the brain."
People who are born or brought up in cities are twice as likely to be schizophrenic as people born into more relaxing settings, according to previous work. The risk for anxiety is 20 percent higher for city-dwellers. And their risk for depression and other mood disorders is nearly 40 percent higher.
To test the biology behind those trends, Meyer-Lindenberg and colleagues challenged dozens of people to solve a series of difficult math problems under time pressure. An adaptive computer program that produced harder problems in response to correct answers ensured that participants could answer only about a third of the questions correctly.
All the while, participants were shown performance meters, which essentially told them: "You're the worst person ever in our lab to do this task," Meyer-Lindenberg said.
But that's not all. "We'd come in between rounds and criticize them. We'd say, 'We know this is difficult for you, but you know, this is important and expensive. Please try harder.'"
Measurements of stress hormones, blood pressure and heart rate confirmed that participants were, indeed, stressed out by the task. More revealing were brain scans taken by fMRI machines.
A part of the brain called the amygdala, which is involved in anxiety, depression and the fight-or-flight response, was extra active in people who were from cities, the researchers report today in the journalNature. What's more, that level of activity went up with the size of the city they were from.
Regardless of where people lived now, results also showed that people who were brought up in cities had higher levels of activity in a region of the brain called the perigenual anterior cingulate cortex (pACC), which regulates the amygdala. And the more years they lived in urban areas as kids, the more active that region was.In people and animals, hyperactivity in the amygdala has been linked to anxiety problems and other mental health disorders.
"What we show is that if you live in a city, those brain areas that are linked to mental illness are hyperactive," Meyer-Lindenberg said.
Future brain-scanning studies should help scientists figure out whether it's noise, pollution, over-stimulation or other factors that give cities such influence over our brains.
"It might be the number of times you encounter strangers or it might have to do with visual stimuli, like billboards," said Dan Kennedy, a neuroscientist at the California Institute of Technology in Pasadena. "We don't have a precise explanation for these effects. But it opens up a new area for research."
More than half of people on Earth now live in cities, and that number is expected to be close to 70 percent by 2050. Reversing those demographic trends is unrealistic. What is possible, suggests the new research, is constructing cities that are healthier for people to live in.
"Humans are now living in environments we really didn't evolve in, and our brains might not be perfectly tuned to these new environments," Kennedy said. "We're facing pressures and situations that our species may have never encountered in the past. I think that's just something we have to keep in mind moving forward as cities develop."