Elsevier

Schizophrenia Research

Volume 35, Issue 3, 15 February 1999, Pages 237-242
Schizophrenia Research

Season of birth and schizophrenia: a systematic review and meta-analysis of data from the Southern Hemisphere1

https://doi.org/10.1016/S0920-9964(98)00139-XGet rights and content

Abstract

Aims: Data from the Northern Hemisphere support an excess of winter–spring births of individuals who later develop schizophrenia when compared with the general population. The data from the Southern Hemisphere have been less consistent. This paper will present a systematic review and meta-analysis of relevant data from the Southern Hemisphere. Methods: To identify relevant studies we searched electronic databases, reviewed citations from target publications and wrote letters to published authors in the field. The counts for observed and expected births were assessed in four planned comparisons. In the absence of significant heterogeneity, the data were combined using Mantel–Haenzel odds ratio in a fixed effect model. Results: Twelve studies were identified. Published and unpublished data from eight of these were able to be included in the analyses. For the two seasonal comparisons (n=20 017), small but non-significant excesses were found in the first comparison (winter versus other seasons; OR=1.04, 0.99–1.08) and for the second comparison (winter and spring versus other seasons; OR=1.03, 0.99–1.07). For the two quarterly comparisons (n=14 799), there was a small but non-significant excess found in the third comparison (third quarter versus other quarters; OR=1.03, 0.98–1.09), and a small but non-significant deficit in the fourth comparison (third and fourth quarter versus other quarters OR=0.99, 0.95–1.04). Conclusions: Assuming that season of birth acts as a proxy marker for fluctuating non-genetic risk-modifying factors for schizophrenia, this review suggests that in the Southern Hemisphere these factors may be weaker, less prevalent, less regular, and/or may be modified by other confounding or modifying variables.

Introduction

Studies undertaken in the Northern Hemisphere have shown a small but relatively consistent excess of winter and spring births of individuals who later develop schizophrenia compared with the general population (Torrey et al., 1997). Jablensky (1995)noted that the season-of-birth effect was one of the most robust findings in the epidemiology of schizophrenia. Despite this feature, much remains unknown about season of birth and schizophrenia. For example, we do not know the nature of the risk-modifying factor, the direction of the effect (i.e., increasing the risk for those born in winter and spring versus decreasing the risk for those born in autumn and winter), or the precise timing of the event (prenatal or early life).

In the latest review of the area, Torrey et al. (1997)identified 10 studies of season of birth from Southern Hemisphere data. The need for more data from the Southern Hemisphere has been emphasised because of a potential artefact called the `age–incidence effect'. In brief, this artefact suggests that those born during the first few months of the year (most of the Northern Hemisphere winter and spring) would be older than those born in later months. Lewis (1989)suggested that this feature would interact with the age-of-onset curve for schizophrenia, resulting in more affected individuals being born in winter and spring. While most commentators dismiss the impact of this factor (Bradbury and Miller, 1985; Torrey et al., 1997), data drawn from the Southern Hemisphere have the advantage of moving the months of interest (i.e., winter and spring; June–November) away from the part of the calendar thought to be at-risk from the age–incidence effect (i.e., January and February).

Hare (1975)emphasised the importance of sample size in season-of-birth studies, reminding us that many small `negative' studies are, in fact, inconclusive studies due to a lack of statistical power. In order to increase our ability to identify non-genetic risk factors of small effect, researchers will need to combine data. The strength of this approach has been demonstrated in recent individual patient meta-analyses examining the impact of obstetric complications and risk of schizophrenia (Geddes et al., 1998). The aim of this study was to undertake a systematic review and meta-analysis of data related to season of birth and schizophrenia drawn from the Southern Hemisphere.

Section snippets

Method

Studies were included in this review if they met the following criteria:

  • 1.

    They included data on season of birth of individuals with schizophrenia (diagnosed by any criterion).

  • 2.

    Data were drawn from countries in the Southern Hemisphere.

  • 3.

    Schizophrenia births were compared to births of well controls (i.e., either population-base birth data or matched well controls).

Four electronic databases (Medline, PsychLIT, Embase, LILACS) were searched for relevant articles using the search string `(schizophrenia

Results

The initial search strategy identified 11 studies which met the inclusion criteria (Krupinski et al., 1976; Parker and Neilson, 1976; Parker, 1978; Syme and Illingworth, 1978; Jones and Frei, 1979; McGrath et al., 1995; Berk et al., 1996; d'Amato et al., 1996; Morgan et al., 1996; Jones et al., 1997; Dalén and Roche, 1998). The only study from South America (Garcia Hildebrandt, 1992) was excluded, because it lacked a general population or well control group (schizophrenia versus dysthymia).

In

Discussion

This meta-analysis of data from the Southern Hemisphere was unable to detect any significant season-of-birth effects for any of the comparisons. The two seasonal comparisons (winter versus other, winter plus spring versus autumn plus summer) showed excess schizophrenia births in the postulated months; however, despite a sample size of over 20 000, this excess did not reach statistical significance. As with all systematic reviews, there is a possibility that small, negative or inconclusive

Acknowledgements

The authors would like to acknowledge the assistance of T. d'Amato, G. Parker, P. Dalén. V. Morgan, D. Castle, P. Joyce, D. Symes, M. Berk, J. Ohaeir, and K. Soares, The Stanley Foundation supported this project.

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    One systematic review of schizophrenia studies found a population attributable risk for being born during winter and spring months of only 3.3% (Davies, Welham, Chant, Torrey, & McGrath, 2003). Reviews of Southern hemisphere data also conclude the effects of season of birth are significant but, if anything, they demonstrate even smaller effects than are apparent in Northern hemisphere data (McGrath & Welham, 1999). In addition to seemingly small effects, one reason contemporary audiences are skeptical of the literature is its general inability to identify a precise mechanism.

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1

Details of this paper were presented at the Winter Workshop on Schizophrenia Research, Davos, Switzerland, February 1998.

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