Manic-depressive psychosis and season of birth.

National series of psychiatric inpatients studied in Scandinavian countries and in England and Wales have all shown that compared with live births in the general population, schizophrenic patients have a significant excess of births in the early months of the year. But there has been disagreement on whether a similar birth distribution holds for manic-depression. The present paper presents new data on the seasonal distribution of births of patients born in England and Wales between 1921 and 1955. Compared with all live births, manic-depression was associated with a significant excess of births in the first quarter, and a corresponding deficiency in the third quarter of the year. Neurotic depression showed no such association. Possible reasons for the disagreement among national findings for manic-depression include difference in the proportion of first ever admissions in the series, differences in age structure, and differences in diagnostic practice and classification. The importance of the age structure of a series is considered in relation to: a) the possible effect of age on the manifestation of a disorder associated with some seasonally related constitutional damage; b) the possibility of secular variation in the severity of a seasonally related noxious factor; and c) the effect of age incidence on distorting the expected seasonal distribution of births in any series of cases.