Relative age effect: a global constant

Age categories in youth football are generally defined from January 1st onwards. Issue number 345 of the CIES Football Observatory Weekly Post analyses the month of birth of 43,938 players fielded in 2021 in 119 leagues worldwide. The study confirms the relative age effect, i.e. the advantage to be born early in the year to make a career.

In total, 31.2% of players in the sample are born in the first trimester of the year. In contrast, only 19.0% are born in the last trimester. Without relative age effect, these proportions should have been around 25%. Per national origin, the stronger concentration of footballers born in the first trimester was measured for the Chinese (43.6%). An over-representation was recorded for 64 of the 67 origins with at least 200 representatives in the sample.

The very low proportion of Japanese born in the first trimester (16.2%) is also the result of a relative age effect. Indeed, in Japan, youth categories start on April 1st. In England, players born in the first trimester are just slightly over-represented (27.1%) insofar as the reference date to define age categories is September 1st.