What is the relative age effect?
The relative age effect is the phenomenon, in sport and education, whereby children born in the earlier months of the year are advantaged over those born later within the same age group. An age group (for example, “children born in 2015”) actually spans almost a 12-month age range. A child born in January is about a year older than a peer born in December; this can mean being taller, stronger and more coordinated.
This small calendar difference creates a large physical difference at young ages. When coaches unknowingly select more mature (and usually earlier-born) children as “talented”, early-born children turn out to be clearly over-represented in groups.
When combined with maturation
The relative age effect grows stronger when combined with a biological maturation difference. A child born at the start of the year who is also an early developer can be several years ahead in maturation compared with a peer born late and developing late. In that case, the observed “superiority” reflects timing and maturity differences far more than true talent.
The result is a systematic error in talent selection: genuinely talented but late-born/late-developing children may be eliminated early because they are physically overpowered. This is a loss both for those children and for the sport as a whole.
What can be done?
There are ways to reduce the relative age effect. One of the most effective approaches is bio-banding, which groups children by maturity; this balances the inequality created by calendar and maturity differences. Another approach is to consciously take month of birth and level of maturation into account in selection and assessment.
Most fundamentally, instead of eliminating children early at a young age, broad participation should be maintained and development thought of in the long term (LTAD). Because the physical superiority observed in childhood is most often temporary; once maturation evens out, true talent emerges.
The reversed effect and the “younger-age” advantage
Interestingly, at the very top level the relative age effect sometimes reverses. Athletes born late in the year, who lagged physically for a long time, are forced to develop more skill, resilience and game intelligence in order to survive. This “hidden advantage” (the underdog effect) is shown in some research as late-born athletes being proportionally more successful at professional level. So an early physical superiority can even turn into a long-term disadvantage by making skill development lazy.
For this reason, the message for families and coaches should be clear: falling behind physically at a young age is not a lack of talent. Keeping a late-born or late-developing child in the system rather than eliminating them early — giving them time and opportunity to develop — is both fair and wise. Assessment that consciously takes month of birth and maturity into account reduces both individual unfairness and the sport’s loss of talent.