Timing is everything: An analysis of the relative age effect in a professional football club

Introduction

The Relative Age Effect (RAE) refers to the advantage that athletes born earlier in the selection year have over their later-born peers. This study analyzed the presence and evolution of RAE in a Spanish professional soccer club, considering sex, competitive level, and playing position.

Material and methods

A cross-sectional and retrospective observational design was applied to a total of 251 male and female players (12 teams) registered during the 2023–2024. Birthdate distributions were categorized into quartiles and semesters and compared with those of the general Spanish population obtained from the National Institute of Statistics. Statistical analyses included chi-square tests, Z-tests for proportions, Cramer’s V, and odds ratios with 95 % confidence intervals, complemented by heatmap visualizations.

Results

A significant RAE in youth categories, with early-born players (Q1 and Q2) overrepresented, especially among female forwards, challenging the notion that RAE is limited to male or physically dominant roles. In contrast, elite teams showed no significant RAE, supporting the “underdog hypothesis,” which suggests that relatively younger players who overcome early disadvantages may develop superior psychological or technical skills.

Conclusions

The RAE is significantly evident in the younger categories, regardless the gender. The overrepresentation of players born in the first half of the year suggests that professionals such as coaches or scouts should consider this bias and implement targeted strategies to mitigate its impact. Practical recommendations include rotating cut-off dates, refining technical-tactical assessments, and supporting late-born players, particularly in female youth teams.

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