Study reveals impact of birth month on football careers

What do footballers Jamal Musiala, Arda Guler and Cristiano Ronaldo have in common? Not only do they star for their respective countries in Germany, they were also both born in February. Researchers from the University of Strathclyde have discovered that this is no coincidence.

A research project at the University of Strathclyde, led by MSc Applied Economics student Aidan Rooney and Dr Markus Gehrsitz, has found strong evidence of a relative age effect in this summer’s European football tournament.

This phenomenon, which also occurs in education and several other sports, concerns the over-representation of individuals born earlier in a cohort.

The study found that there are almost twice as many January-born players in the squads as December-born players, even though both months have similar birth rates. Overall, there is a clear decline in the representation of later-born players compared to those born in the first few calendar months of the year.

Age advantage

Researchers call this phenomenon the “relative age effect.” Because most soccer confederations use January 1 as the eligibility date for their youth teams, players born in January are nearly a year older than players born in December in the same group. Their initial age advantage can translate into a greater chance of being selected for their national youth teams.

The Strathclyde researchers confirm that the relative age effect is indeed stronger at youth level. Their analysis of the U17 teams from the European Championships earlier this year revealed that more than four times as many players were born between January and March than between October and December. Under-17 champions Italy are a case in point; the 20-man squad included 11 players born in the first quarter of the year and just one player born in December.

Lead researcher Aidan Rooney explains: “One year may not seem like a big difference, but at youth level it could mean that players born in January have had almost 10% more playing time than those born in December.”

However, the initial advantage eventually seems to level off. The study examined the correlation between birth month and player market value, a proxy for player performance and quality. Among players, birth month was not a significant predictor of market value.

Dr Gehrsitz said: “The age advantage is huge at youth level and an early birth month still increases the chance of making the national team at adult level. But once a player reaches that level, other factors determine whether they become a superstar.”

Main influence

There is debate about how exactly birth month affects a player’s chance of selection. Physical development seems to be an important influence and the study findings support this channel. Previous research also points to psychological factors, social skills and playing experience.

One must also be careful not to generalise. What is true on average is not necessarily true in individual cases. Kylian Mbappe (December), Pedri (November) and Ilkay Gundogan (October) are world class in their own way and are among the best players in Europe, despite being born in the last months of the year.

Whatever the mechanism, an early birth month still helps prospects get a foot on the career ladder. An earlier-born youth player within a certain age bracket is likely to stand out more due to their likely greater height, strength, and physical, mental, and emotional maturity. As such, they may be offered more coaching opportunities, increasing their chances of going pro and leading to the skewed distribution we see in the professional game.

In their professional careers, especially during international tournaments, attributes such as height become less noticeable, making them less noticeable in the broader competitive field.

At what might be considered the pinnacle of sport, athletics is a given. True elite status then requires distinguishing factors such as exceptional skill, remarkable speed or a consistent ability to score.

Stand out

Aidan Rooney said: “Our findings are very interesting and, although initially surprising, they make a lot of sense and paint a clear picture of the role birth month plays in all of this.

“The bottom line is this: if a young player is born early in the year, it greatly increases the chance that he will stand out within his age group and be picked up. That increases his chances of becoming a professional and possibly later being selected for the national team.

“But these advantages only go so far. At the elite level, success depends on a number of other, more important factors. So it is not the case that the best players are the early ones.”

The study will be submitted for publication and peer review in the fall.