Regional variation in the incidence of schizophrenia in Finland: a study of birth cohorts born from 1950 to 1969

BACKGROUND: We investigated whether there is regional variation in the incidence of schizophrenia and if so. whether it is caused by urban-rural differences, larger spatial clustering, or both. To control for the effect of migration, we examined regional variation in the incidence according to place of birth. METHODS: Finnish birth cohorts born from 1950 to 1969 were followed in the National Hospital Discharge Register from 1969 until 1991, and all cases of schizophrenia (ICD-8 or ICD-9 295) were identified (N = 14828). Forty-eight of the 559 municipalities were classified as urban and 25% of the Finnish population lived in these municipalities in 1960. For the analysis of spatial clustering, municipalities were grouped into 57 functional small-areas. We used Poisson regression model with the number of births of individuals who later developed schizophrenia as a response variable, and place of birth (urban/rural), birth cohort (1950-54, 1955-9, 1960-64, and 1965-9), functional small-area units, and sex as response variables. RESULTS: The incidence was slightly higher among the rural-born in the oldest birth cohort. In the other cohorts, it was higher among the urban-born, and the difference between urban and rural born increased in the youngest cohorts. Significant spatial clustering of schizophrenia was observed in eastern Finland. CONCLUSIONS: Urban birth is a risk factor for schizophrenia in Finland in cohorts born since 1955. However, genuine spatial clustering of schizophrenia in eastern Finland was also observed, possibly caused by genetic isolation.