Psychiatric patients have been shown to differ from the normal population in important features of early development. Numerous clinical investigations reviewed by Bowlby3have indicated that the development of mental illness may be traceable to conditions and events occurring early in life. Two statistical studies1,2have shown a relationship between maternal bereavement during the first few years of life and the subsequent development of psychiatric illness. A number of other statistical studies, summarized later in the present paper, have tested whether psychotic patients differ from the normal population in the distribution of months of birth. One reason why such studies have been done is undoubtedly the ease of collecting the data. Mental hospitals routinely attempt to record the date of birth of every patient; the accurate month of birth is usually obtainable from the patient or a close relative. The monthly