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Season of birth in schizophrenia: no latitude at the equator

Institute of Mental Health, Singapore and School of Psychiatry, University of New South Wales, Australia

RATHI MAHENDRAN, FAMS and ENG SENG KOH

Woodbridge Hospital, Singapore

DAVID MACHIN, PhD

NMRC Clinical Trials and Epidemiology Research Unit, Singapore

Correspondence: Professor Gordon Parker, Institute of Mental Health, 10 Buangkok View, Singapore 539747

Declaration of interest Funded by the Institute of Mental Health, Singapore.

Background If the established winter excess in births of people who subsequently develop schizophrenia is an effect of ‘seasonality’, this would be testable by examining the pattern of births in an equatorial region with no formal seasons.

Aims To investigate whether there is any variation in month of birth among patients from equatorial Singapore with a diagnosis of schizophrenia.

Method All 9655 patients discharged from Singapore's national psychiatric hospital with a diagnosis of schizophrenia were included (year of birth range 1930-1984). We analysed aggregated data, as well as the data of subsamples grouped according to birth-year periods, in order to examine secular trends. One patient subsample (those born 1960-84) allowed exact matching against the general population data set and close testing of any seasonal influence.

Results Monthly variation in births was evident for both patients and controls; the patterns were very similar, apart from the patient sample showing a trough in March-April.

Conclusions In an equatorial region, where ‘seasons’ are absent, no seasonal excess in births of those later developing schizophrenia was evident.