The association between season of birth and the risk for schizophrenia.

Many epidemiologic studies have been conducted to discover factors that might bear on the origins of schizophrenia. In general, the results of these studies have been contradictory. One consistent finding, however, is an association between season of birth and the risk for schizophrenia. This paper reports a test of the hypothesis that season of birth is associated with the risk for subgroups of the schizophrenic population. The results of regression analyses of data from Monroe County, New York, are reported. All white residents who were reported to have been diagnosed schizophrenic for the first time during the period January 1, 1969, to December 31, 1971, and who were hospitalized at least one day in the five-year period following the first schizophrenic diagnosis formed the group of schizophrenic patients. The population data used for the denominators of the incidence rates consisted of all white residents living in Monroe County, New York, as of April 1, 1970. An effect of month of birth on the risk for schizophrenia was related to the sex and the age of subsets of the whole population.