Abstract
The Relative Age Effect (RAE) describes systematic differences in athlete representation according to birth date, but it remains unclear whether the associated birth-quarter asymmetries vary across geographical contexts or reflect a broadly shared international pattern. This study examined birth-quarter distributions associated with the RAE in senior international football using a cross-sectional sample of 1248 players from the 48 national teams participating in the 2026 Fédération Internationale de Football Association (FIFA) World Cup. Birth-quarter distributions were assessed using χ2 goodness-of-fit and independence tests, and a cumulative link mixed model evaluated the effects of age, height, playing position, and continental confederation, with national team included as a random effect. Marginal probabilities by age, spatial mapping of first-half-year births, and exploratory unsupervised cluster analysis were also performed. Birth-quarter distribution differed significantly from an equal reference distribution, with greater representation of players born in the first birth quarter (Q1: January–March) and second birth quarter (Q2: April–June) and lower representation in the fourth birth quarter (Q4: October–December). Playing position and confederation were not significantly associated with birth quarter, whereas player age was the only covariate that remained statistically significant in the mixed-effects model. Older age groups showed a lower representation of players born in Q4. Low between-team variance and weak cluster separation indicated limited differentiation in birth-quarter distributions among the national rosters analysed, although descriptive variation in first-half-year (H1: Q1 + Q2) representation was evident. These findings are consistent with an asymmetric birth-quarter distribution at the senior international level; however, because talent identification and development systems were not directly evaluated, these observations cannot establish specific organizational mechanisms or a uniform structural bias.