No evidence of time trends in the urban–rural differences in schizophrenia risk among five million people born in Denmark from 1910 to 1986

Abstract

Background. There have been conflicting reports on time trends in urban–rural differences in the incidence of schizophrenia. This study explored the potential time trends in these differences with regard to birth cohort and age at onset.

Method. Linking data from the Danish Civil Registration system with data from the Danish Psychiatric Central Register, a cohort born in Denmark from 1910 to 1986 was established (5·05 million people). Overall, 23051 people were classified with schizophrenia in 1970–2001.

Results. Urban–rural differences in schizophrenia risk may have existed for people born in Denmark since 1910, and have existed at a constant level for people born from 1945 to 1986. Males aged <20 years had a risk of 3·90 [95% confidence interval (CI) 3·28–4·65] associated with urban birth while males [ges ]20 years had a risk of 2·12 (1·98–2·27). Females <20 years had a risk of 2·49 (95% CI 2·01–3·09) associated with urban birth while females [ges ]20 years had a risk of 1·90 (95% CI 1·74–2·08). At age 46, 1·84% (95% CI 1·76–1·93) of males and 1·05% (95% CI 0·99–1·12) of females born in the capital area had developed schizophrenia, while 0·81% (95% CI 0·75–0·86) of males and 0·56% (95% CI 0·51–0·60) of females born in the rural area had developed schizophrenia.

Conclusions. There was no evidence of time trends in the urban–rural differences in the incidence of schizophrenia in Denmark, suggesting that the cause(s) responsible for these differences were not related to exposures that became more prevalent in urban areas over time. This finding is in contrast to findings from Finland and The Netherlands.