Cross-national and historical variation in relative age effects in professional football.

The relative age effect (RAE) - the overrepresentation of players born early in the selection year - is one of the most robust findings in sports science, yet how much it varies across countries and over time remains surprisingly underexplored. Drawing on birth-date data for 49,578 male professional footballers across 44 national systems and five birth cohorts (1985-2009), this study describes three patterns that have not previously been examined together. RAE magnitude varies enormously across national football systems, ranging from close to zero to birth-quarter gaps exceeding 40 percentage points. It has also increased across successive birth cohorts: players born in 2005-2009 show birth-quarter imbalances roughly 9 percentage points wider than those born in 1985-1989. Common macro-level indicators - national footballing strength, gross domestic product (GDP) per capita, and population - show at most weak and inconsistent associations with cross-national differences in RAE and do not account for the historical increase once country and cohort are held fixed. Together, these findings provide a comparative benchmark for understanding cross-national and historical variation in relative age effects.