Schizophrenia Daily News Blog

Does season of birth affect schizophrenia?

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

Messias E, Kirkpatrick B, Bromet E, Ross D, Buchanan RW, Carpenter WT Jr, Tek C, Kendler KS, Walsh D, Dollfus S.
Arch Gen Psychiatry. 2004 Oct;61(10):985-9.

Schizophrenia is an illness due to interactions between genes and environmental factors. Although it has been debated by researchers, one of the environmental risk factors suggested is season of birth. Winter birth was first reported to be a risk factor for schizophrenia in 1929 and many other studies have replicated this finding. Researchers have suggested that family history and the prevalence of certain viruses (eg Borna Disease Virus seropositivity), seasonal variations in infectious agents (eg viruses), sunlight exposure and vitamin D, and the availability of nutrients are all explanations that could explain this seasonality of birth effect noticed in schizophrenia.

Some researchers have gone on to further suggest that the clinical features associated with winter births differ from those of �deficit schizophrenia�. Deficit schizophrenia is defined with respect to the lack of positive symptoms, where a person has mainly negative symptoms (eg. lack of emotion, lack of pleasure etc) and a relatively severe form of the illness. But other researchers have suggested that there are more summer births in patients with deficit schizophrenia. Since it is unclear whether summer or winter births affect this type of �deficit� schizophrenia, this study looked at pooled data from the previously published studies as well as unpublished data in order to clarify the role of season of birth in schizophrenia.

The researchers looked at studies of season of birth in which it was possible to make a deficit/nondeficit categorization based on clinical scales/questionnaires. They selected published studies with samples of convenience and all known population-based studies with the deficit/nondeficit categorization. The studies came from 6 countries including the United States, Ireland, England, Scotland, Spain and France. They ended up extracting 3 published studies of samples of convenience, 2 population-based prevalence studies, and 5 population-based studies that approximated incident samples.

They found that there was a significant difference between deficit and nondeficit people in season of birth and that this difference was largely due to an increase in deficit schizophrenia births in June and July. From this, they concluded that deficit schizophrenia has a season of birth pattern that differs from that of nondeficit schizophrenia. And the authors suggest that deficit schizophrenia may be a separate disease within the syndrome of schizophrenia.

It is important to note the limitations of this study. First, data was only collected from northern hemisphere countries. An unpublished multi-center prevalence study by a group from Australia has failed to show an association between deficit schizophrenia and summer birth. It is also difficult to decide from this study whether the association between deficit schizophrenia and summer births is due to an increase in summer births in the deficit group compared with the general population or a decrease in summer births in the nondeficit group. Third, although this season of birth effect is one type of hypothesis proposed, it is by no means written in stone. There are many researchers who argue against this idea. More research is needed to clarify this relationship.

This study was supported in part by grants MH44801 (Dr Bromet), MH41953 (Dr Kendler), MH40279 (Dr Carpenter), and MH60487 (Dr Messias) from the National Institutes of Health, Rockville, Md.

Click here to find this article on PubMed

Author: Farzin Irani