On Relative Age Effect

Recently, while I was chatting with my fellow swimmer, he stunned me with the statement that his younger son (born in February) is much more likely to succeed in swimming then his older son (born in December) because of his birth month.

He explained his claim with "Relative Age Effect" (RAE)

In most sports, youth categories are determined by calendar year children were born in. While it makes perfect sense that someone born in January 2010 will have more likely been selected for basketball team then someone born in December 2010 (article image might give you a hint why), it is not so apparent why RAE persists over time.

Yet, the evidence of RAE being present in senior categories is compelling.

Here is the graph of UEFA U17/U19/U21 football players participating in 2010/2011 football competitions (blue bar is actual distribution, while red bar is expected distribution based on general population):

Effect is very evident even among Italy Series A footballers:

RAE is present in most of the sports. Here is distribution of Beijing Olympics 2008 male athletes per quarters per sport

Median birth date was June 10, much before July 2 of general population.

RAE effect varies among sports. Here is a summary from few different studies:

• RAE effect is much stronger in male then in female sports
• Collective sports (football, basketball, handball) as well as swimming and athletics show prominent RAE effect
• Sports where competitors are divided in categories based on competitor weight (like boxing or wrestling) do not show RAE effect at all
• In gymnastics, weak reverse RAE effect is observed (younger competitors in the same age group are more prevalent). This is probably because height/maturity is detrimental in this sport

Common explanation to why RAE stays in effect in senior categories is heritage from youth categories. Since older members of cohort are more likely to be selected for competition, they are more likely to receive additional training (more training slots, campuses, official competition etc). Younger members of cohort are more likely to quit sports. They perceive they are not good enough, since they didn't make it to the team, although it might be that they are more gifted then the older members of the age group.

If you are world class in sports, RAE can significantly influence your paycheck. Here is comparison of wages of Series A players born in Q1 and Q4

RAE extends beyond sports.

One study found statistically significant, albeit low, RAE effect in academics. Children born in first half of the year scored on average 1.2 points more on tests than children born in H2 in the 4th grade, and 0.8 points more in the 8th grade. Each observed year, H2 children were marginally less likely to attend university from H1 children.

Thus, in academics, although present, RAE seems to diminish over time much faster than in sports.

Key takeaway

If you have a talented child who is into sports/science, born near the end of the year, don't let it be disappointed when he/her does not make it to competition squad.

Let her/him read this article instead :)