Schizophrenia, season of birth, and maternal age.

Maternal age is a neglected variable, with obvious implications for aetiology, in psychiatric research. In samples of schizophrenic patients, the distributions by month of birth and maternal age at birth show remarkable anomalies. These phenomena may be connected, because the seasonal distribution of births varies with maternal age, probably as a result of the age-dependent changes of fertility. Preliminary attempts to describe the seasonal effect in schizophrenia as a function of the shift in mean maternal age are not wholly successful at the quantitative level. Southern-hemisphere findings do not seem to fit the predicted pattern. Examples from other areas of research are discussed (sudden unexpected death in infants, congenital dislocation of the hip, and handedness) in which the seasonal distributions of births, and the findings on maternal age, conform to different varieties of the expected relationship.