Urbanization and psychosis: a study of 1942–1978 birth cohorts in The Netherlands

Background. Urban birth is associated with later schizophrenia. This study examined whether this finding is diagnosis-specific and which individuals are most at risk. Methods. All live births recorded between 1942 and 1978 in any of the 646 Dutch municipalities were followed-up through the National Psychiatric Case Register for first psychiatric admission for psychosis between 1970 and 1992 (N=42115). Results. Urban birth was linearly associated with later schizophrenia (incidence rate ratio linear trend (IRR), 1·39; 95% confidence interval (95% CI), 1·36–1·42), affective psychosis (IRR, 1·18; 95% CI, 1·15–1·21) and other psychosis (IRR, 1·27; 95% CI, 1·24–1·30). Individuals born in the highest category of the three-level urban exposure were around twice as likely to develop schizophrenia. Associations were stronger for men and for individuals with early age of onset. The effect of urban birth was also stronger in the more recent birth cohorts. Conclusions. There are quantitative differences between diagnostic categories in the strength of the association between urban birth and later psychiatric disorder. High rates of psychosis in urban areas may be the result of environmental factors associated with urbanization, the effect of which appears to be increasing over successive generations.