A total of 1814 patients were studied from the Mississippi State Hospital with a DSM-III diagnosis of schizophrenia and aged 15-39 years at the time of admission. The 634 schizophrenics with a birth date between 1 December and 31 March were compared with the 1180 born between 1 April and 30 November for age of admission, race, sex, marital status, duration of initial admission and clinical subtype. The only significant difference between the variables was the duration of the first admission: winter-born patients had a shorter psychiatric hospitalization than summer-born schizophrenics. These findings are discussed in accord with similar studies.

Prior reviews indicate that schizophrenics tend to be born in the winter, relative to non-psychiatric controls. This conclusion has been criticized, however, as the association between birth seasonality and schizophrenia may be the result of a statistical artifact, the age-incidence effect. To examine this possibility, we studied the birth seasonality of 2892 schizophrenics, controlling for the age-incidence effect. Both before and after instituting these controls, we found excesses for the months of December and March. We conclude that the age-incidence hypothesis does not provide any general explanation of the season-of-birth effect in schizophrenia.