The Link Between REA, Selection Bias, and Injury in Youth Sports

Portugal’s Rodrigo Mora in action with Italy’s Alessandro Di Nunzio REUTERS/Yiannis Kourtoglou

Talent identification and development are crucial for long-term success in youth sports, yet subtle biases can significantly impact outcomes. The relative age effect (RAE) occurs when children born earlier in a selection year, often tied to cut-off dates, gain physical, cognitive, and emotional advantages over later-born peers. First observed in education, this effect now significantly influences sports, determining who joins teams, enters elite academies, and advances to higher levels of competition. Selection bias exacerbates this challenge, as coaches tend to favor athletes who appear more advanced due to their age and physical maturity. However, these factors also contribute to injury risks, especially when selected athletes face intense training loads that don’t match their biological maturity. 

“The RAE and selection bias play a major role in shaping youth sports…”

Understanding the Relative Age Effect

The RAE leads to an overrepresentation of athletes born just after a sport’s age-group cut-off date. For example, in soccer or hockey, where cut-offs often align with January 1 or September 1, those in the first quartile (Q1) can be nearly a year older than their peers in the fourth quartile (Q4) in the same category. This gap provides advantages in size, strength, speed, and coordination during the formative years, boosting selection rates for early-born athletes. For example, 44.4% of youth soccer players from a sampled group were born in the first quartile, clearly showing this trend (see figure 1)⁽¹⁾.

Biologically, RAE is closely tied to maturation. Early born children are more likely to experience advanced pubertal development, giving them a competitive edge. This effect varies by gender and sport, with stronger impacts in male youth sports⁽¹⁾. In elite academies, RAE can linger into adolescence, though it may fade at senior levels as late maturers who survived deselection catch up. Socially, RAE affects participation, as later-born athletes often face deselection or drop out due to the difficulty of competing against more mature peers⁽²⁾.