The Relative-Age Effect in Male Japanese Football: A CrossSectional Analysis from Youth to Senior Competitive Level

Abstract

The objective of this study was to examine the magnitude and persistence of the Relative-Age Effect (RAE) in competitive Japanese youth and senior football. A cross-sectional analysis of birthdate distributions of Japanese male youth academy football players (n= 4,488) from the U12 (n=268), U13 (n=481), U14 (n=464), U15 (n=483), U18 (n=1059) and professional players (n=1733) in the 2022 season was conducted. Chi-square goodness of fit tests and Cramer’s V tests were carried out to reveal the incidence and effect size of the RAE across the age categories. A significant RAE (P< .001) was found. It became more pronounced with age, peaking in the U18 age group (Cramer's V= 0.436; Q1/Q4 OR 6.04) before declining in senior professional players (Cramer's V= 0.204; Q1/Q4 OR=2.23). The organisation of youth competition in Japan, with a transition from annual age groups up to U15 to the U18 age group representing three years (U16, U17, and U18) appears to increase the selection bias towards early born players to the detriment of those born later in the age group. However, the transition to senior football saw, compared with the U18 proportions, a reduced proportion of Q1 players, a very similar proportion of Q2 players and increased proportions of Q3 and particularly Q4. The clear implication is that a Q3 and Q4 youth player represents a better prospect for senior professional football than Q1 or Q2 and Q2 than Q1.