Gender-specific changes since 1900 in the season-of-birth effect in schizophrenia.

Background: It was hypothesised that the size of the season-of-birth effect may have decreased in tandem with the apparent decline in the incidence of schizophrenia.

Method: Through the Aberdeen Psychiatric Case Register, subjects were identified who had been diagnosed as schizophrenic and had been born between 1900 and 1969. The ratio of winter/spring to summer/autumn births was compared across the seven decades for both sexes together, for men, and for women.

Results: For the 1935 men, but not for the 1620 women, there was a highly significant increase (P = 0.0009) in season-of-birth effect.

Conclusion: Non-seasonal factors have contributed to a declining incidence of schizophrenia in both sexes. 'Seasonal' factors to which female foetuses are more susceptible than male foetuses (such as infection or malnutrition) have also decreased in frequency, severity, or both, but this has not happened with factors affecting males, leading to an increase of their season-of-birth effect.