New research suggests that people living in large cities are more likely than small-town residents to exhibit psychotic-like symptoms such as paranoia and delusions. In a study of more than 7,000 people in the Netherlands, investigators found that both full-blown psychotic disorders and milder psychosis-like symptoms were more common among those living in urbanized areas. The link was most apparent among people born and raised in densely populated areas, suggesting that childhood environment may create ?enduring liabilities? for adult psychiatric health, according to Dr. Jim van Os of Maastricht University and his colleagues, who reported their findings in the July issue of the Archives of General Psychiatry.

They divided the study participants into groups living in one of five levels of urbanization, based on population density. Through interviews, van Os and his colleagues determined that psychiatric symptoms increased along with participants? level of urbanization.

In the overall population, the prevalence of any type of psychotic symptoms over a lifetime is about 4% to 17%, depending on the symptoms. The investigators looked at the number and severity of symptoms such as feelings of persecution or auditory hallucinations. Only about 1% suffered mental illnesses such as major depression, and even fewer had been affected by schizophrenia-like disorders.

The fact that such symptoms were more common in urban settings could be partly explained by symptomatic individuals having ?drifted? to these areas, the authors note. However, they add that their previous research has shown that around 75% of city-dwellers were born there.

A number of studies have demonstrated that psychotic illness is more prevalent in urban settings than in rural areas. ?The high rates of psychotic illness in urban environments may be the result of the influence of environmental factors,? van Os and colleagues write. ?As the urban effect appears to have its impact during urban upbringing rather than during adult residence per se, developmental mechanisms ought to be considered.? The fact that in this study, psychosis-like symptoms as well as full-blown illnesses were more common in cities suggests a greater ?psychosis proneness? in urban settings.

Why this connection exists is unclear, but the researchers note that deprivation and social isolation in childhood neighborhoods have been shown to affect mental health. Another recent study in Denmark linked urban birth with an increased of developing schizophrenia. While family history was a far greater predictor of the risk, place of birth emerged as an environmental factor. Those researchers speculated that people born in urban areas are exposed to more infections during prenatal development and childhood. Some experts believe early exposure to infections may affect the developing brain in a way that makes it more vulnerable to schizophrenia.

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