Cobley et al. 2009, Annual age-grouping and athlete development: meta-analytical review of relative age effects

Sports Medicine 39(3):235–256. Meta-analysis of 38 studies, 253 independent samples, across 14 sports and 16 countries, examining the prevalence of the relative age effect (RAE) in sport. Finds consistent prevalence of RAE across youth sport contexts, with adolescent males competing at representative level in highly popular sports showing the strongest effects. The effect size increased linearly with the magnitude of relative age differences. Concludes that direct policy and practitioner intervention is required to address RAE-driven inequalities in athlete development. Strong.