Relative Age Effect Badminton: Why Birth Month Skews Squads

Last checked: August 2026 · Written by the player-led team at Badminton House

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How the Relative Age Effect Happens in a Selection Year

The relative age effect is the advantage a child gets from being born early in a sport's selection year rather than from being more talented. Youth categories commonly group players by the calendar year they were born in, so a January-born player can be selected alongside a same-category teammate born the previous December, up to about twelve months younger.

January 1 is the cutoff date used by most age-grouped sports to form these yearly cohorts. A 12-year-old born on January 3 and a 12-year-old born on December 29 the year before can end up in the same "under-13" group, even though one of them has had almost a full extra year of height, reach, coordination and general physical maturity to develop. Neither player did anything to earn that gap. It is a side effect of how the cohort was drawn.

That gap matters most exactly when it is hardest to see: before and during puberty, when a year of growth can change a child's height, strength and coordination far more than a year of extra coaching would. A coach watching a trial session sees who wins the rally, not who was born eleven months earlier. The birth-month advantage gets read as talent, and the roster fills up with players who happened to be born in the right quarter.

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The Evidence: Birth-Month Bias Across Sports and in Badminton

Birth-month bias in youth sport selection is not a badminton quirk. A meta-analysis of 38 studies across 14 sports found the pattern consistently, and a 2024 nationwide analysis of Turkish racket-sport players found the same skew inside badminton specifically, with early-year-born players clearly over-represented in both sexes.

The wider evidence comes from a 2009 meta-analysis that re-examined 38 separate studies published between 1984 and 2007, covering 253 independent samples across 14 sports and 16 countries. It found a consistent relative age effect overall, though the size of the effect was usually small and grew larger the bigger the age gap being compared. The sports and groups most at risk were adolescent boys aged 15 to 18, competing at regional or national representative level, in sports with a lot of participants. That combination (young teenage years, competitive selection, a popular sport) describes plenty of badminton pathways.

For badminton specifically, a nationwide analysis of Turkish racket-sport players looked at 17,134 registered badminton players and found a clear skew toward early-year births in both sexes. Among female badminton players, 28.7% were born in the first quarter of the year (January to March) against 19.8% born in the last quarter (October to December). Among male badminton players the split was 29.3% versus 19.1%. Across all racket sports in the study, players born at the start of the year were 1.63 times more likely to be represented than those born at the end of it, and the skew was statistically significant in badminton, table tennis and tennis, though not in squash.

Treat any single number here as a snapshot of one country's data, not a universal constant. The same paper notes that earlier badminton-specific studies found no significant relative age effect at all, and a separate study found one only among the female players who competed at the 2008 Beijing Olympics. The direction of the bias (favoring early-year births) is consistent across the badminton research that exists, but its size clearly depends on the country, the competition level and the years studied, so a specific percentage from one federation should not be read as "the" badminton figure.

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Why an Early Physical Edge Does Not Predict the Best Player

An early size or strength advantage in a 10-year-old says almost nothing about who will be the strongest adult player, because growth timing varies by years between individual children. Talent-identification research finds that testing children once, before or during puberty, is likely to exclude late-maturing players who show real promise.

A widely cited review of talent identification programs put it plainly: cross-sectional testing, where every child is measured once against the same standard, is "likely to exclude many, especially late maturing, 'promising' children from development programs" because sporting talent is dynamic rather than fixed. The review's own recommendation is to build programs that track maturity status over time and keep potential in view, instead of screening children out early based on a single test day.

A separate line of evidence shows what happens to that early advantage as players get older. A study of Italian men's national football teams tracked 885 players from Under-16 through senior level across two decades. Less than 20% of senior national-team players had ever been selected for the Under-16 team, and only 9% of players picked for Under-16 went on to earn a senior call-up. The birth-quarter skew that drove Under-16 selection was large (players born in the first quarter of the year were about 8.3 times more likely to be selected than those born in the last quarter) but it shrank to roughly 3.3 times by Under-19. The study's own conclusion was that junior national-team selection is an unreliable marker of future success, specifically in that sport and dataset.

That finding is football, not badminton, and it should not be stretched into a badminton claim. What it does illustrate, alongside the exclusion problem above, is the general shape of the risk: the physical edge that gets a child selected at 11 or 12 has far less predictive power by the time players are competing as adults, and a selection system built entirely around who wins the most points today will keep mistaking that fading edge for talent.

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Three Ways Clubs and Coaches Can Correct for It

Coaches and clubs cannot change when children are born, but three concrete changes reduce how much birth month distorts training and selection: group some sessions by physical maturity rather than birth year, delay any selection decision that cannot be undone, and track growth so nobody mistakes a growth spurt for a skill gain.

1. Group some training by maturity, not just birth year. For drills and conditioned games where size matters (net play under pressure, smash defense, physical rallies), put players who are at a similar stage of physical development against each other, even if their ages differ by a year or more. A technically sound but smaller player gets to test real skill against a peer of similar reach, instead of being physically overpowered by a bigger same-age opponent. This is exactly what talent-identification researchers recommend when they say maturity status should factor into how programs are run, not just chronological age.
2. Delay any selection decision that cannot be reversed, and keep the door open. A club that permanently sorts 10 and 11-year-olds into an "elite" squad and a "recreational" group based on who is winning that season is locking in an error that the evidence above says will mostly wash out within a few years. Run squads as open rosters with a regular re-entry point, treat early selection as provisional, and make it genuinely easy for a late-developing player to move up once their game catches up to their growth.
3. Track height and growth at intervals, not just skill. Measuring height every few months (or simply asking parents to flag a recent growth spurt) gives a coach the context to read a match correctly. A player who suddenly looks clumsy might be mid-spurt, not regressing. A player who is winning easily on size alone is not automatically the best technician in the group. Tracking growth turns a guess about "who's talented" into an informed read of "who's ahead physically right now, and why."

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What a Parent Should Ask a Program

Before enrolling a child in a competitive badminton pathway, a parent can ask four direct questions that reveal whether a program accounts for the relative age effect: how it groups training, whether selection decisions are reversible, how it treats late developers, and how it re-evaluates players over time.

1. "Do you ever group by size or stage rather than just age?" A program that never mixes age groups for any drill is testing every child against the same growth-stage assumption.
2. "If my child isn't selected this year, what's the path back in?" A real re-entry route matters more than the initial selection decision.
3. "How do you talk about players who are currently smaller or later to develop?" Listen for whether the coach frames a smaller player as behind or simply as not yet grown.
4. "How often do you re-test or re-rate players?" A program that rates a child once at age 9 and never revisits it is treating a snapshot as a verdict.

None of these questions require a coach to be a researcher. They require a program to admit that a birth certificate is not a skill test, and to have already built a process around that fact rather than discovering it the day a talented, smaller December-born player quits.