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Relative age effect


The term relative age effect (RAE) is used to describe a bias, evident in the upper echelons of youth sport and academia, where participation is higher amongst those born early in the relevant selection period (and correspondingly lower amongst those born late in the selection period) than would be expected from the normalised distribution of live births. The selection period is usually the calendar year, the academic year or the sporting season. The difference in maturity - which can be extreme at young ages: a six-year old born in January is almost 17% older than a six-year old born in December in the same year - causes a performance gap that persists over time.

The term month of birth bias is also used to describe the effect and season of birth bias is used to describe similar effects driven by different hypothesised mechanisms.

The bias results from the common use of age related systems, for organizing youth sports competition and academic cohorts, based on specific cut-off dates to establish eligibility for inclusion. Typically a child born after the cut-off date is included in a cohort and a child born before the cut-off date is excluded from it.

The most commonly used cut-off date for youth international sporting competition is 1 January. The IOC and FIFA and the 6 international football confederations (AFC, CAF, CONCACAF, CONMEBOL, OFC and UEFA) all use 1 January as their administrative cut-off date when determining an athlete's eligibility to compete in youth competitions, children born before a specified cut-off date are excluded.

Malcolm Gladwell's book Outliers: The Story of Success and SuperFreakonomics by Steven Levitt and Stephen Dubner have popularised the issue in respect of Canadian ice-hockey players, European football players and US Major League baseball players.

The expected distribution of births in any given month across a population correlates closely to the number of days in the month, with February as the shortest month having the fewest births. The first graph shows the distribution of births, by month, for the European Union over the ten years from 2000 to 2009. There is a slight but clearly perceptible increase in the birth rate in the summer months.


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