Tuesday, June 23, 2009

Cutting Edge Research: Age Effect At The U17 World Cup

Is there an advantage to being born in the early part of the year? The relative age effect means that if you are born close to the cut-off age for a competition; you are more likely to be selected for the team than if you are born later in the year. This phenomenon seems to occur at all levels of play from local travel teams to the most advanced youth competitions. A new study published in the Scandinavian Journal of Medicine and Science in Sports shows that in nearly all countries participating in the U17 World Cup, there is a strong relative age effect. In fact, nearly 40% of the players are born in the first three months of the year while only 16% are born in the last quarter.

In this study, the rosters of all teams participating in the last six U17 World Cup competitions were obtained from FIFA. The player birthdates were recorded and analyzed to determine of there was a trend for players to be born in one part of the year versus another.

The first figure shows the percent of players born each month. It’s easy to see that there are far more players born in the early part of the year than in the later. This trend holds for all six competitions, and for all of the FIFA regions except Africa (more on this later). Also, the most and least successful teams showed similarly strong age-effects.

Several previous studies into the relative age-effect show that in many youth sports, it is advantageous to be born near the cut-off date for a give competition. For international soccer competitions (e.g. U17 World Cup), that cut-off date is based on the calendar year. For American youth soccer, that date is associated with the American school year, July 31.

Studies show that the relative age-effect most often occurs because coaches who select players for advanced training or competition typically use physical and psychological maturity as a major indicator of performance. Thus, those players with early birthdates tend to be bigger and faster than those born later in the year. As discussed previously on the Science of Soccer Online, this may have dramatic consequences for player development (click here). Potential players with late birthdates are being overlooked at a young age simply because they are not as physically mature. They are then denied the developmental opportunities that their older counterparts are offered. As a result, the talent pool for national teams, especially at the U17 level, may be diminished.

Back to the African countries... The African region showed a much different patters of birth month distribution that the other regions. For the African teams, in particular, the western African countries of Ghana, Nigeria and Togo, showed a reverse age effect. That is, more players were born in the later part of the year than in the early months. 14% of the African players were born in December. Also of all underage players participating in the tournament, 41% were on African team rosters. The second figure clearly shows the spike in African birth dates in December of the competition year. Contrary to other regions, in Africa, it seems to be more advantageous of be born in the late in the year rather than early.

It’s not clear why the African teams show this trend. One explanation is that the African teams manipulate birth certificates in order to make older players eligible for competition. It is also possible that there are legitimate errors on African players’ birth certificates. According to UNICEF, many African births are not registered. That is, many African children are not issued a birth certificate when born. For many kids, birth certificates are issued years after birth. In these cases, parents often guess the actual birthdates. So, it’s not surprising that some player’s birthdates might be clustered in a single month.

The bottom line, even at the highest level of youth competition, the FIFA U17 World Cup, the relative age effect exists. Whether this is the result of long-term trends in the player selection process or the training environment provided for older players is not known. In either case, there are several negative outcomes for excluding potentially exceptional players based on their relative age and maturity.


Williams JH (2009) Relative age effect in youth soccer: analysis of the FIFA U17 World Cup Competition. Scandinavian Journal of Medicine & Science in Sports. In press, DOI:10.1111/j.1600-0838.2009.00961.x