Objective: Based on inconsistent findings in the literature, this study tested the hypothesis that "there is a season of birth bias for females with anorexia nervosa (AN)."
Method: Females with AN, born in 1975 to 1996, were compared to females born in the same years and geographical regions by chi-square test for contingency tables with known population parameter testing for monthly deviations. Five groups were based on a priori power calculation and geographical location: Iceland, Norway and Sweden (N = 847), United Kingdom (N = 706), Oregon, USA (N = 394), Argentina and Brazil (N = 486), and Australia (N = 381).
Results: The hypothesis was not supported in any of the groups. The associations (Cramér's V) between month of birth and the differences in distributions of births ranged from 0.05 to 0.08, none of which were statistically significant.
Discussion: The main implication of these findings is that season of birth may not play a significant part in the aetiology of AN.
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