Risk factors and confounders in the geographical clustering of schizophrenia.

In response to previous reports of geographical clustering of individuals with psychosis, a study was carried out in Denmark utilizing the national case register. Two-thousand one-hundred and ninety-nine (2199) individuals with schizophrenia were divided by place of birth into 217 geographical areas and analyzed by age, gender, month of birth, genetic relatedness, and degree of urbanization of birthplace. Heterogeneity was ascertained using log-linear Poisson regression. The greatest amount of heterogeneity was associated with degree of urbanization of place of birth. Heterogeneity was also associated with age-gender interaction and calendar period. When adjusted for these factors, there was no remaining heterogeneity, suggesting that all geographical clustering in our study was explained by the above factors.