Season of birth and schizophrenia: sex difference.

1. A statistically significant increased risk of schizophrenia for individuals born in winter has been reported. The increase risk is of the order of 5-15%. The seasonal effect is more marked among females. This winter birth effect suggests some environmental agents, probably a neuropathogen one, acting on the foetus. 2. The present study sought to test the environmental damage hypothesis by application of the family history of psychiatric disorder distinction to season of birth data divided according to sex and using a control population. 3. From computer records, all patients admitted to the psychiatric department of Marseilles Timone hospital between January 1984 and December 1989 who satisfied DSM III, DSM III R criteria for schizophrenia were identified. Patients were then classified into two groups : family history of psychiatric disorder versus no family history. Division according to the sex was carried on after two groups were formed. 4. The data show (1) a significant excess of births in the early months of the year (p < 0.05) for all patients with no family history of psychiatric disorder, (2) a significant excess of births for females (p < 0.05) with no family history. 5. These results provide indirect support for the neurodevelopmental theory of schizophrenia.