The Relative Age Effect in Sports: Why Birth Month Shapes Who Gets Picked

When the advantage fades: the underdog question

The rugby review discusses what researchers call the underdog hypothesis. Relatively younger players who survive early selection may, through years of competing at a disadvantage, develop stronger technical and psychological skills, while the anthropometric and physiological edge of the relatively older may prove transient once growth evens out.

The data are consistent with part of that story: the effect was weaker in senior than in youth rugby. But it persisted into senior levels, and a smaller adult effect could also reflect who stays in the sport. Treat the hypothesis as a plausible explanation the authors discuss, not as a finding that late-born players end up better.

Fixes, and how strong the evidence is

Present age information where the eye can use it. Mann and van Ginneken asked talent scouts from an elite football club to watch junior games and rank players on potential. Scouts with no age information showed a selection bias towards the relatively older, and the bias remained when they were given players' birthdates. When scouts knew that shirt numbers corresponded to relative age, the bias was eliminated. The limits matter: it was one controlled study with one club's scouts, and it measured rankings rather than careers. It does suggest that knowing a birthdate is not the same as adjusting for it while watching.

Bio-banding. Instead of grouping by birth year, bio-banding groups adolescents by estimated biological maturity, most often by the percentage of predicted adult height they have reached. A 2026 systematic review by Han and colleagues found 13 studies involving 861 youth soccer players; nine estimated maturity with the Khamis–Roche predicted-adult-height method. In bio-banded games, late-maturing players showed better decision-making and composure and more involvement in winning the ball, while early maturers were less dominant. The review's own verdict is cautious: the evidence relies on short-term match responses and proxy outcomes, not direct evidence of more accurate selection or better long-term development. Maturity is also a different measurement from birth date, since a player can be relatively young and early-maturing or the reverse, so bio-banding targets the maturation side of the problem rather than relative age as such.

Other interventions. Adjusted cut-off dates, wider age bands, birth quotas and average-team-age rules have also been tried. The studies cited here do not evaluate them, so this article offers no verdict on them.

Reading a youth pathway with this in mind

Football Career Soccer XI Sim runs a fictional footballer's career from youth football through the academy, university football and turning professional, with minutes earned through Coach Trust and attributes such as pace, strength, passing and vision developed along the way. Nothing in the game's description says it models birth dates or physical maturation, so there is no birth-month mechanic to look for. What the research offers is a way of reading any youth pathway, real or simulated.

Judge the trajectory, not the snapshot. A ranking taken at 13 mixes ability with timing. The same caution applies at the other end of a career, where ageing curves rather than birth dates shape what a single season shows.

Small tilts compound across stages. Suppose, purely for illustration, that Q1 players were 1.2 times as likely as Q4 players to clear each of three selection stages. After all three, the ratio would be 1.2 × 1.2 × 1.2 ≈ 1.73: a modest per-stage bias becomes a large one at the top of the funnel. That is the same multiplication of probabilities across independent steps described in understanding probability in sports simulations, and it fits the pattern of larger effects at more selective levels.

A pool of survivors hides the filter. Draft games such as 24-0 Rugby Draft Pro League build their boards from real players' careers across eras, with ratings that are the game's own estimates. Every name on such a board already made it through youth selection; the players the filter screened out never reach a board, which is one reason a pool of finished careers can make early talent look easier to spot than it was. What makes a good draft board covers evaluating the players who are there.

Guides to the rules and research behind each sport are gathered on our sports hub, and the studio's career and management games are listed under sports GM.