Nyårsbarnet: Både för- och nackdelar vara januaribarn

The term relative age effect (RAE), also known as birthdate effect or birth date effect, is used to describe a bias, evident in the upper echelons of youth sport and academia, where participation is higher amongst those born early in the relevant selection period (and correspondingly lower amongst those born late in the selection period) than would be expected from the normalised distribution of live births. The selection period is usually the calendar year, the academic year or the sporting season. The difference in maturity - which can be extreme at young ages: a six-year old born in January is almost 17% older than a six-year old born in December in the same year - causes a performance gap that persists over time. The term month of birth bias is also used to describe the effect and season of birth bias is used to describe similar effects driven by different hypothesised mechanisms. The bias results from the common use of age related systems, for organizing youth sports competition and academic cohorts, based on specific cut-off dates to establish eligibility for inclusion. Typically a child born after the cut-off date is included in a cohort and a child born before the cut-off date is excluded from it.