These findings suggest that SoB is a risk factor for AN. However, none of the studies have been methodologically satisfactory. Future research needs to overcome numerous methodological challenges and to explore specific hypotheses to explain this bias.

The main implication of these findings is that season of birth may not play a significant part in the aetiology of AN.

There appears not to be any season of birth bias in women with bulimia nervosa in Norway.

BackgroundOutcomes from studies on season of birth bias in eating disorders have been inconsistent. This inconsistency has been explained by differences in methodologies resulting in different types of effect sizes. The aim of the current study was to facilitate comparison by using the same methodology on samples from two studies with differing conclusions.MethodsThe statistical analyses used in each study were applied to the samples from the other study and the resulting effect sizes, Cramêr’s V and odds ratio (OR), were compared and discussed.ResultsFor both studies, the Cramêr’s Vs ranged between 0.03 and 0.08 and the OR ranged between 0.85 and 1.31. According to common conventions, Cramêr’s Vs below 0.10 and ORs below 1.44 are considered small.ConclusionAs a marker of one or more potential risk factors, the observed effects are considered to be small. When reanalysed allowing for direct comparisons, studies with contrasting conclusions converge towards an absence of support for a season of birth bias for patients with AN.