Mental Disorder and Season of Birth

Abstract

MOST investigations of the relationship between season of birth and mental illness have found that a disproportionate number of patients with schizophrenia or manic-depression were born in the early months of the year^1,2. Most of these studies can reasonably be criticized on the grounds that the samples of patients were (a) not representative of all patients, and (b) not comparable with the general population used as the basis of expectation, because seasonal distribution of births in a general population may vary significantly not only from year to year but between adjacent geographical areas in the same year³. Dalen's study⁴ largely avoided these criticisms because his schizophrenic patients were representative of the whole of Sweden; he made a decade-by-decade comparison (P. Dalen, personal communication) of years of birth with the general population. He found a very significant excess of the patients had been born in the months January to March.