Early risk factors in schizophrenia: place and season of birth.

First admission psychiatric patients born in England and Wales between 1938 and 1963, and discharged from hospitals in England and Wales between 1976 and 1986, were examined. Using logistic regression, we tested the hypothesis that the risk of shizophrenia varies by place, and season of birth. Persons born in city areas showed a 12% greater risk of schizophrenia (odds ratio 1.12; 95% confidence interval 1.06 to 1.19) than those born in non-city areas, when compared with other psychiatric patients. The increase in risk was particularly high for individuals born in city areas in winter (21%, ie odds ratio 1.21 and confidence interval 1.08 to 1.36). These findings suggest that the factor(s) responsible for the season-of-birth effect preferentially affects city born schizophrenics.