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

Aims: Data from the Northern Hemisphere support an excess of winter-spring births of individuals who later develop schizophrenia when compared with th e general population. The data from the Southern Hemisphere have been less consistent. This paper will present a systematic review and mete-analysis o f relevant data from the Southern Hemisphere. Methods: To identify relevant studies we searched electronic databases, reviewed citations from target p ublications 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 Published and unpublished data from eight of these were a ble to be included in the analyses.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 compa rison (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-significa nt excess found in the third comparison (third quarter versus other quarter s; OR=1.03, 0.98-1.09), and a small but non-significant deficit in the four th 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 f or fluctuating non-genetic risk-modifying factors for schizophrenia, this r eview 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. (C) 1999 Published by Elsevier Science B.V. All rig hts reserved.