Season of birth: comparison of patients with schizophrenia, affective disorders and alcoholism.

The distribution of patients suffering from schizophrenia, affective disorders and alcoholism by months of their birth was studied, all patients having been reliably diagnosed using Research Diagnostic Criteria. Significant differences were found between the three groups. The winter-spring birth rate excess in schizophrenia was confirmed, a spring-summer birth rate excess in alcoholics was demonstrated. Different distributions are due to different reasons: for schizophrenic patients the harmful effects hypothesis and for alcoholics the procreational hypothesis seem to be appropriate explanations.