Season of birth in eating disorders : fact or fiction

ABSTRACT A season of birth bias is said to be present if the season of birth pattern for a particular group varies from the pattern within the local normal population. Such a phenomenon may contribute to the aetiological understanding of psychological disorders, perhaps by identifying subgroups within heterogeneous diagnostic categories. This can have implications for treatment. For example, season of birth biases have been identified for subgroups within the eating disorder category, especially for patients with anorexia nervosa (AN). This dissertation will review the findings from season of birth research in eating disorders and summarize some methodological issues. Two proposed hypothesis that aim to explain this bias, will be explored. As an additional contribution to the field, one of those will be developed further. Directions for future research to expand this field of knowledge will be offered throughout. Despite methodological issues that need to be addressed, the reviewed papers consistently show a season of birth bias for AN. Here is a brief presentation of the two hypotheses that will be explored: The first hypothesis claims that the fertility of some thin, food restricting women will vary as a consequence of temperature fluctuation through the year. This is due to a higher need to use energy to maintain bodily temperature in colder months. Conception is more likely in the warmer months, leading to spring births. Encouragement and modelling of restrictive eating will increase the child s vulnerability for AN. The second hypothesis proposes that it is variations in sunlight exposure that lead to the observed bias. This could happen because; i) Intense ultraviolet radiation might cause maternal oxidative stress or mutations, both which may alter the neurodevelopment in some foetuses. This leaves them vulnerable for AN. ii) Higher levels of sunlight exposure might alter the mothers mood by lifting symptoms of depression. This may improve her sexual life, increasing likelihood of conception in the summer. iii) The interaction between the mother-child might be altered as a consequence of a higher birth weight in the spring due to increased sunlight exposure during the first trimester of gestation. The interaction increases the child s vulnerability to AN. Based on this discussion of the methodological issues, discussion and development of these hypotheses, future research can settle if the SoBB in ED is a fact or fiction

Comparing effects: a reanalysis of two studies on season of birth bias in anorexia nervosa

Background Outcomes 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. Methods The 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. Results For 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. Conclusion As 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