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= Urbanicity and schizophrenia onset =

In the Global North, the risk of developing schizophrenia is affected by the degree of urban exposure. Estimates show that the risk of developing schizophrenia in the most urban environment is 2.37 times higher compared to rural areas in developed countries. The nature of this relation between schizophrenia and urban areas has been a matter of investigation since the 1930s. Scientific efforts that could explain this increased incidence follow the drift or shift hypotheses.

Incidence & risk
Schizophrenia is a mental disorder characterised by continuous or relapsing episodes of psychosis. Currently, around 20 million people are diagnosed with schizophrenia worldwide. Urbanicity is a risk factor for schizophrenia onset. However, most research has been done in the Global North, the effect of urban areas on schizophrenia onset in developing countries (Global South) requires more research.

In developed countries, living in the most urban environment is associated with a 2.37-fold risk of developing schizophrenia, compared to rural areas. Besides the rural-urban difference, the increased urban risk of schizophrenia onset in the Global North also depends on:


 * The degree of urbanisation. In a Danish study, individuals living in a higher degree of urbanisation than 5 years earlier had a 1.40-increased risk compared to if they did not move.
 * The timing of moving to an urban area during childhood. In a Danish study, individuals that moved between their 10th and 15th birthday to a more densely populated area, had a 1.4-fold risk of developing schizophrenia compared to those who did not.
 * Urban upbringing. In a Danish study, those who had always been living in a capital had a 2.62-fold risk compared to people who were raised in rural areas and did not move.

In general, the more years lived in a city with a higher degree of urbanisation, the greater the risk of schizophrenia onset in developed countries.

Early work
The relation between urban lifestyle and mental health has been one of the oldest topics within psychiatric epidemiology. In 1939, Mental Disorders in Urban Areas, Faris and Dunham mapped the number of first mental hospital schizophrenia admissions by Chicago neighbourhoods (see fig. 1). They found that the incidence of first mental hospital admissions for schizophrenia was greater in the city center of Chicago, with decreasing rates once you move to the periphery. The finding that schizophrenia is more common in more urban deprived neighbourhoods has been replicated many times among cities in the Global North. Within the psychiatric-epidemiological literature, the cause of this relationship is up for debate ever since, with scientific evidence supporting both the drift hypothesis and breeder hypothesis.

Proposed mechanisms
The fact that schizophrenia is more common in cities in developed countries, sparked debate on why this is: does the city make people ill, or do ill people move to the city. In this article, two hypotheses are discussed: the drift and breeder hypotheses.

Drift hypothesis
According to the social selection drift hypothesis, the increased number of schizophrenia cases in the cities in developed countries is due to the selective migration based on their health. Following the drift hypothesis, the increased number of schizophrenia cases could be explained by direct and indirect selection.

By direct selection, patients with schizophrenia move to or from a specific area and remain there. So, patients need to move too or remain in the city for mental care facilities, which results in a higher number of schizophrenia patients within those cities.

By indirect selection, people at risk for developing schizophrenia move to the city or remain there. In a Swedish study, they found that people who were more genetically prone to develop schizophrenia moved to poorer neighbourhoods, before the onset of the disease.

Breeder hypothesis
According to the breeder hypothesis, the increased incidence of schizophrenia in the Global North is due to the landscape of the city itself. This is due to the spatial variation in the exposure to environmental factors and/or spatial variation in health behaviours.

Much of the work on environmental exposures that increase the risk of developing schizophrenia has been focused on prenatal infection and exposure. For example, traffic-related pollutants during pregnancy increase the child’s risk for developing schizophrenia, as well as viral infections of the mother during pregnancy, which spreads faster in cities. Alternative explanations could also be sought in the social environment of the city. In areas with high competition and greater wealth disparities, such as cities, disadvantaged individuals are hypothesised to be more vulnerable to develop schizophrenia due to social defeat.

Certain health behaviours increase the risk of schizophrenia, and are more common in cities in developed countries. For example, escalation of adolescent marijuana is a risk factor for schizophrenia onset and more common in urban areas in developed countries.