Mental disorder and season of birth--a southern hemisphere study.

Studies on the relationship between season of birth and mental disorder have been substantially confined to northern hemisphere regions. Such studies have generally found an excess of winter births of schizophrenics, and variably an excess of winter births of manic-depressive and mentally retarded patients. In the present study information on sex, diagnosis and date of birth was obtained on all 20,358 patients first admitted to psychiatric facilities in New South Wales between July 1970 and June 1974 and born in New South Wales. The collective 1962-71 monthly live-births for New South Wales were used as a control. A significant winter excess was found for the female schizophrenic group, while a significant spring excess was found for neurotic patients, most marked in those with anxiety neurosis. It is hypothesized that the relationship between schizophrenia and winter birth is consequent upon a greater sensitivity of schizophrenics to those physiological factors which determine conception in the general population.