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 = 14 828). Forty-eight or 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.