Relative Age Effect

The relative age effect, also known as the birthdate effect or birth date effect, describes a bias observed in the upper echelons of youth sport and in academia, in which participation is higher among those born earlier in the relevant selection period, usually the calendar year, the academic year or the sporting season, than would be expected from the underlying distribution of births, and lower among those born later in the period. Differences in maturity often contribute to the effect, with age category, skill level and context also affecting its size; mid-to-late adolescent, regional-to-national, popular contexts see the highest risk, while under-11, recreational, less popular contexts see the lowest. The effect is distinct from month-of-birth or season-of-birth bias, which examines prenatal and perinatal seasonal environmental factors such as sunlight, temperature or viral exposure and their relation to health outcomes; the relative age effect instead shifts with the selection date itself, so that children born soon after a cutoff date are typically included and those born soon before it excluded.