Season of birth and schizophrenia: a systematic review and meta-analysis of data from the Southern Hemisphere.
OBJECTIVE
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 Published and unpublished data from eight of these were able to be included in the analyses.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.
McGrath
J J
JJ
Queensland Centre for Schizophrenia Research, Wolston Park Hospital, Wacol, Australia. jjm@brain.wph.uq.edu.au
Welham
J L
JL
D001315:Australia / Q000453:epidemiology; D001723:Birth Rate; D006801:Humans; D012559:Schizophrenia / Q000453:epidemiology*; D012621:Seasons