Summer birth and deficit schizophrenia: a pooled analysis from 6 countries.

BACKGROUND In some reports, summer birth has been associated with deficit schizophrenia. Deficit schizophrenia and nondeficit schizophrenia also differ in several other ways. OBJECTIVE To conduct a combined analysis of the published and unpublished data sets from the northern hemisphere that relate deficit and nondeficit schizophrenia to month of birth. DATA SOURCES Studies of season of birth in which it was possible to make a deficit/nondeficit categorization. STUDY SELECTION Published studies with samples of convenience and all known population-based studies with the deficit/nondeficit categorization were included. The studies came from 6 countries. DATA EXTRACTION Three published studies of samples of convenience, 2 population-based prevalence studies, and 5 population-based studies that approximated incident samples were included. Month of birth was compared for deficit and nondeficit schizophrenia, using meta-analytic fixed-effects models. DATA SYNTHESIS A group x month goodness-of-fit chi2 showed a significant difference between deficit and nondeficit subjects in season of birth (P < .001) in the studies that approximated incidence. This difference was largely due to an increase in deficit schizophrenia births in June and July (odds ratio, 1.9; 95% confidence interval, 1.3-2.9). Similar results were found in the prevalence studies. A similar pattern was found in 2 of the 3 samples of convenience, but when combined, these 3 samples did not show a significant deficit/nondeficit difference. CONCLUSIONS Deficit schizophrenia has a season of birth pattern that differs from that of nondeficit schizophrenia. This analysis supports the notion of a separate disease within schizophrenia.