Season of birth in schizophrenia and affective psychoses in Western Australia 1916-61.
OBJECTIVE: This study examines seasonality birth effects in schizophrenia and affective psychoses in the southern hemisphere, given possible confounding of age-incidence effects with winter birth peaks in northern hemisphere data. METHOD: Distributions of births by season, quarter and month for individuals born in Western Australia 1916-61 with a diagnosis of schizophrenia (N=2284), affective psychoses (N=3236), and neurotic depression (N=4869) on the statewide mental health register were compared with distributions for the general population. RESULTS: We found no association between season of birth and schizophrenia, affective psychoses or neurotic depression. For schizophrenia, the pattern of risks by quarter reflects northern hemisphere trends. Results by month are difficult to interpret due to large fluctuations in the data. While age-incidence effects had no impact on the distribution of risk, we found an artefactual increase in January births due to routine imputation of missing birth dates. CONCLUSION: Adjusting for artefacts in the data produced a pattern analogous to northern hemisphere trends.