Background. The urban environment may increase the risk for psychotic disorder in interaction with pre-existing risk for psychosis, but direct confirmation has been lacking. The hypothesis was examined that the outcome of subclinical expression of psychosis during adolescence, as an indicator of psychosis-proneness, would be worse for those growing up in an urban environment, in terms of having a greater probability of psychosis persistence over a 3·5-year period.

Method. A cohort of 918 adolescents from the Early Developmental Stages of Psychopathology Study (EDSP), aged 14–17 years (mean 15·1 years), growing up in contrasting urban and non-urban environments, completed a self-report measure of psychotic symptoms at baseline (Baseline Psychosis) and at first follow-up around 1 year post-baseline (T1). They were again interviewed by trained psychologists for the presence of psychotic symptoms at the second follow-up on average 3·5 years post-baseline (T2).

Results. The rate of T2 psychotic symptoms was 14·2% in those exposed to neither Baseline Psychosis nor Urbanicity, 12·1% in those exposed to Urbanicity alone, 14·9% in those exposed to Baseline Psychosis alone and 29·0% in those exposed to both Baseline Psychosis and Urbanicity. The odds ratio (OR) for the combined exposure was 2·46 [95% confidence interval (CI) 1·46–4·14], significantly greater than that expected if Urbanicity and Baseline Psychosis acted independently.

Conclusion. These findings support the suggestion that the outcome of the developmental expression of psychosis is worse in urban environments. The environment may impact on risk for psychotic disorder by causing an abnormal persistence of a developmentally common expression of psychotic experiences.