The season of birth of siblings of psychiatric patients.

The season of birth of the siblings of psychiatric patients was studied to test the hypothesis that an unusual seasonal pattern of maternal conception is the cause of the excess of births in the early months of the year which has been found in national studies of psychotic patients. Information on the month of birth of the siblings was obtained from interviews with psychiatric in-patients born in Britain. The quarterly distribution of births of 670 mentally well siblings of psychotic patients (schizophrenia and manic-depression) was significantly different from that of 1,513 siblings of other psychiatric patients and also from that of all births in England and Wales. The results lend qualified support to the hypothesis, but they were not clear-cut and there was some evidence that the information on birth dates was biased.