An association between month of birth and method of suicide

Abstract

Background. A statistically significant association between season of birth and suicidal behaviour has been reported. However, the effect of month of birth on the choice of suicide method is yet to be established. Aim. This study examines the association between commonly used methods of suicide and season of birth using data on suicide collected over a 21-year period in England, Wales and Scotland. The sample size available, in excess of 52,000 suicides, greatly exceeds all previous studies in this field. Method. Data on suicides registered between 1979 and 2000® were obtained from the Office for National Statistics (ONS) for England and Wales, and the General Register Office (GRO) for Scotland. Our analyses include all suicides [ICD 9 codes; E950– E959] and undetermined injury deaths [E980–E989], reported between 1979 and 2000 in England, Wales and Scotland for persons born between 1941 and 1966. We used Poisson and negative binomial generalised linear models (GLMs) with seasonal harmonic components. Results. Adjusting for the year of birth, the model predicts that the average increase in risk of suicide between the trough (October) and the peak (May) of the seasonal component is 17.9% (95% CI= 13–33%). For males the estimated increase in risk was 15% (95% CI 5–22%) and for females 27% (95% CI 8–47%). The effect of month of birth on suicide applied to all commonly used methods, with the exception of suicide by burning (SBB) with a significant increase of 16% (95% CI 2–37%) in people born in January compared to other methods. Conclusion. Our results replicate our earlier finding of an association between season of birth and suicide incidence. Birth rates of persons who kill themselves show a disproportionate excess in spring compared to other months. The unexpected observed finding in suicide by burning (SBB) may represent the effect of latitude and warrants further examination. Seasonality of birth in suicide may enhance our understanding of some biological aspects in its aetiology and give directions for future research.