Abstract
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.