Paris and southeast London have the highest rate of people reporting psychotic episodes, said an international study on Wednesday that compared rates of the mental disorder in six countries.
A total of 17 areas in Britain, France, Italy, The Netherlands, Spain and Brazil were covered in the report in the Journal of the American Medical Association (JAMA) Psychiatry.
Researchers described the study as the largest international comparison of psychotic disorders to date, and the first major analysis of its kind in more than 25 years.
A previous study in 1992 that spanned eight diverse settings in rural and urban India, Japan, Europe and North America found that the rates of schizophrenic disorders were "surprisingly similar."
But the latest study found that rates of psychosis can be close to eight times higher in some regions compared to others, with the lowest incidence seen in the area around Santiago, Spain, and the highest in inner-city Paris and Southeast London.
"It's well-established that psychotic disorders, such as schizophrenia, are highly heritable, but genetics don't tell the whole story," said lead author James Kirkbride, professor of psychiatry at University College London.
"Our findings suggest that environmental factors can also play a big role."
The study was based on people aged 18-64 who contacted mental health services after a suspected first psychotic episode.
A total of 2,774 cases were analyzed.
Population density was not a factor in the psychotic rate, nor could differences be explained by age, sex or ethnic composition.
Rather, researchers found higher rates in younger men, among racial and ethnic minorities, and that "the strongest area-level predictor of high rates of psychotic disorders was a low rate of owner-occupied housing," said the report.
The findings suggest "social deprivation" may be at play, said co-author Hannah Jongsma, a researcher at the University of Cambridge.
"People in areas that are socially deprived may have more social stresses, which could predict psychosis incidence, as suggested by other studies," Jongsma said.
"An alternative explanation could be that owner-occupied housing is an indicator of social stability and cohesiveness, relating to stronger support networks."
According to an accompanying editorial in JAMA, the study, like the 1992 one before it, "raises more questions than it answers."
"We hope that it will spur further international efforts to explore how variation in sociocultural environments might be associated with psychosis incidence."