Football is a sport that spans cultures around the world, one that transcends conflicts, one that anyone, in some form or another, can play. But the beautiful game is not an equal-opportunity sport, at least not at the highest level.
Research by Dave Oleson and Greg Laughlin of Statwing found that players born earlier in the calendar year were significantly more represented at the 2014 World Cup than those born in the latter months. In fact, there were approximately half as many players born in the month of December as those born in January.
The reason for this disparity is what is called the relative age effect (RAE). As Oleson cites, it is a phenomenon that also has been observed in the National Hockey League (via Journal of Quantitative Analysis in Sports), Major League Baseball (via Slate), and English grade school test scores (via The Institute for Fiscal Studies, UK).
RAE is a fundamentally simple phenomenon. In football, young players are placed by age. For example, those born between January 1 and December 31 of 1995 are eligible for the under-19 European Championship currently being played in Hungary.
A player born in early January is almost a full year older than one born in late December. And although this reality is of little importance for athletes in their mid-20s and up, it can have a huge effect on teenagers.
During the teens, even a few months' more or less physical, mental and emotional development and technical training can make a huge difference on a player's perceived quality. Those born in October, November and December in many cases will fall through the cracks of what is an imperfect system that leaves them at a relative disadvantage.
In the United States, academy football is structured somewhat differently than it is in the majority of the world abroad. But the relative age effect is still observed in the American game, with two peaks due to there being two cutoff dates. In US Youth Club Soccer league (the first step for young athletes), the cutoff is in August. But the US Soccer Development Academy adheres more to international standards, with a cutoff in January.
Accordingly, among under-16 players in the latter, prevalence of players born in January is much higher than that of those born in July, but August birthdays bring a new peak that is almost as high as March.
One can say that the presence of a second cutoff date in August gave those born in the fall a better chance of making their way to the professional stage, but even so, the June and July-born players would have to have overcome the same bias that November- and December-born players face in Europe.
Huge disparities exist in any case, with January-born American under-16 players represented in three times the number of their December-born counterparts.
Oleson and Laughlin note in their analysis that by the time players reach the senior level, some who had previously relied on their advanced maturity had dropped out of the picture as international-class footballers. Normalizing the World Cup birth dates statistics for a 23-man squad, the average team at the World Cup would bring 13.5 players born in the first half of the year.
Based on birth-rate statistics, one would expect an average of 11 (birth rate is higher in the second half of the calendar year) members of a World Cup squad to be born between the beginning of January and the end of June.
Assuming no intrinsic difference in the quality of footballers born in one month versus another, on average, 2.5 extra players per team born in the first half of the calendar year "artificially" benefited from their early birth date. They aren't necessarily more naturally gifted than those later-born players who previously dropped off the scene, they just were granted more opportunities.
In their analysis, Oleson and Laughlin then conduct a thought experiment in which the "artificially-selected" players are replaced by those born in the second half of the year, with the July-to-December-born additions equally likely to be the best, second-best, third-best...(etc) 22nd-best or 23rd-best player on the team.
Statistically, that would mean that each newly added player would have a one-in-23 (4.3 percent) chance of being the best player in the team, a three-in-23 (13.0 percent) chance of being in the top three and a 11-in-23 (47.8 percent) chance of being in the starting lineup (positions notwithstanding).
Expanding to 2.5 players, there would be a 10.8 percent chance of having a new best player, a 32.6 percent chance of a new top-three player and 65.2 percent chance of a new top-six player.
The Statwing analysts therefore conclude that the Relative Age Effect had a big impact on US Soccer at the World Cup, and that if not for it, the USMNT could have fared better. They assert that based on the odds, a 32.6 percent chance of there being another Tim Howard, Clint Dempsey or Michael Bradley if not for the RAE would be a big help for American soccer.
But at the end of the day, there is no clear way of overcoming the RAE and even so, there still is a big gap between the talent pool available to the USMNT as opposed to Germany, for example. The key to American success, Oleson and Laughlin assert, is that rising athletes like Kobe Bryant and Wes Welker grow up loving soccer instead of basketball and football.