[EN] The relative age effect (RAE) has primarily been investigated in male athletes involved in
popular sports and high-level competitions. However, occurrence of RAE in other types of
sports at the grassroots level, particularly in female athletes, is less well-studied. Thus, we
examined the RAE in a large cohort of young athletes who participated in all competitive
sports in Bizkaia, Spain, according to gender and specificity of the sport. The birth dates of
38,381 participants (65.1% males and 34.9% females) aged 9–14 years old in 37 competitive
sports were analyzed. Birth dates were divided into four birth-quarters and compared to
those of all children born in the same period using a χ2 goodness-of-fit test and standardized
residuals. The effect size Cramer’s V was measured, and odds ratio and 95% confidence
intervals were calculated to determine the odds of athletes born in January playing in the
highest leagues. In the total sample, in boys RAE was evident in football, but only in highercompetition
leagues (p<0.001, large effect size). In girls, RAE was evident in the most popular
team sports: basketball (p<0.001, large effect size in basketball 1st league), handball and
football (p<0.05, both small effect sizes). Players born in January were 3.23- and 2.89-times
more likely to play in the 1st leagues than those born in December, for boys (football) and
girls (basketball) respectively. In the overall analysis and in the remaining sports, presence
of RAE was negligible. Therefore, the date of birth does not seem to be a constraint to participating
in most sports in Bizkaia. The potential mechanisms for RAE are multifactorial and
complex, yet a combination of factors, such as the popularity of a sport and the depth of
competition, physicality and social influences may be involved. We discuss these mechanisms
and potential measures to mitigate RAE.This work was partially supported by the
University of the Basque Country (UPV/EHU)
(https://www.ehu.eus/es/) under grant PPGA19/53
and the Basque government (https://www.euskadi.
eus/gobierno-vasco/inicio/) under grant IT922-16.
The funders had no role in study design, data
collection and analysis, decision to publish, or
preparation of the manuscript
