More than half of the world’s population – 4.2 billion people –live in cities. This number is expected to increase, with 68% of the world’s population estimated to live in urban areas by 2050.

Among the world’s megacities, defined as urban areas with more than 10 million inhabitants, Tokyo, Japan is the largest, with 37 million inhabitants. It is followed by Delhi, India (29 million) and Shanghai, China (26 million). In the United Kingdom, after several decades of rural exodus, 83% of the population lives in urban areas – and London has become the first European megalopolis.

The adverse effects of urban life on physical health have long been recognized, including higher rates of cardiovascular and respiratory disease. More recently, however, is the revelation that city life can also have adverse effects on mental health.

The factors that increase the risk of mental illness are neither intrinsic nor inevitable aspects of urban life. Instead, they are the result of poor planning, design and management and could be reversed. »

the risk of developing depression– the world’s most common mental disorder, characterized by low mood and feelings of helplessness – is 20% higher among city dwellers than among those living outside the city. During this time, the risk of developing psychosis– a severe psychiatric disorder associated with hallucinations, delusions, paranoia and disorganized thinking – is 77% higher among city dwellers than rural dwellers. the risk of developing generalized anxiety disordera state of mind characterized by a feeling of anxiety and a feeling of imminent danger or panic, is also 21% higher among urban dwellers than among rural dwellers.

Importantly, the more time you spend in an urban environment during childhood and adolescence, the higher your risk of developing mental illness in adulthood. This “dose-response” association provides indirect support for a causal relationship between urban living and mental illness.

brain science

Support for these epidemiological findings comes from brain science. In a pioneering study in 2011, researchers measured neural activation during a stress-inducing task.

As expected, all participants showed increased neuronal activation in the Limbic system– a network of regions that plays a key role in our daily regulation of emotions. Within this network, neuronal activation in the tonsil– the “fight or flight” center of the brain – correlated to the size of the city in which an individual resided at the time of the experiment. And neural activation of perigeneal anterior cingulate cortex– a region involved in the processing of social stress – correlated with the duration of a participant’s residence in a city during childhood.

Curiously, other studies showed similar alterations in people at high genetic risk for developing psychiatric disorders. This supports the idea that urban life leads to changes within a network of regions implicated in the development of mental illness.

Rio de Janeiro, one of the world’s megacities. [Photo: Flickr user Ed Johnson]

Underlying factors

Together, epidemiological and neuroscientific studies provide converging evidence that indeed, people who live in urban areas are at higher risk for mental health problems. So what specific factors within the urban environment increase the risk of developing such problems?

Epidemiological studies identified a large number of factors. Some of them highlight potential problems in the built environment, such as the reduction access to green spaces and high levels of noise and air pollution. Others relate to the social environment, such as solitudeperceived and actual crimes, and social inequalities.

These studies were based on collecting a single snapshot per participant and therefore could not capture the multiple and diverse environments that most people experience throughout the day. But some new studies are using smartphone technologies to collect multiple metrics as people go about their daily lives. Urban spiritfor example, is a citizen science project that uses a smartphone app to measure the experience of urban or rural life in real time.

It is important to recognize that factors in the urban environment that increase the risk of mental illness are neither intrinsic nor inevitable aspects of urban life. Instead, they are the result of poor planning, design and management and could be reversed. Which brings us to the next question: Could city life be good for our mental health?

The bright side

The incidence of depression in urban areas is lower when people have access to quality housing and green spaces.

While existing research focuses on the negative impacts of urban living on mental health, portraying the accelerated urbanization taking place around the world as a challenge to humanity, this is an oversimplification of what it means to live in a city for at least three reasons.

First, urban life is a complex, contradictory and difficult to define phenomenon, with little in common between the inhabitant of a disadvantaged suburb and that of a garden city; or between the processes of gentrification and those of the decline of city centres. Consistent with this notion, the incidence of depression in urban areas is lower when people have access to quality housing and green spaces.

Second, we know that all health, and mental health in particular, is both nature and nurture. For example, emerging evidence from epigenetics, which examines how the environment affects the expression of our genessuggests that the impact of city life depends on our pre-existing genetic makeup.

Third, for many people, urban living can bring great mental health benefits by increasing opportunities for education, employment, socialization, and access to specialized care. Moving to a city can be the first step towards realizing its full potential and a necessary condition for gaining access to communities with similar interests and values.

Ultimately, cities offer a multitude of obstacles and opportunities, of freedom and captivity, which can challenge and nurture us, often at the same time.

Andrea Mechelli is a professor of early mental health intervention at King’s College London. This article is republished from The conversation under Creative Commons license. Read it original article.