So, the analysis I made was the following: I took the 929 soccer players list who are playing now in first division in Argentina and I counted them according to their month of birth. The result was stunning:
Futbolistas argentinos de primera 2015 por mes de nacimiento
Data source:
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Please note how first-trimester players double fourth-trimester player. Quantity of players who were born in December: 48. Players who were born in January: 126. I mean: just one month away, there are almost triple soccer players.

Discarding some of the logical explanations

After obtaining this information, I looked for evidence that could let me discard this was a random fact, so I went 10 years back and got the same result:
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What other objections could there be? Maybe, there are more births at the beginning of the year. I got this data from the government:
Nacidos vivos por mes - 2011

Source: Health Statistics and Intormation Direction. National Ministry of Health

So, the monthly difference is not a determinant factor.

For some reason, if you are born at the first months of the year, you are more likely of becoming a professional soccer player.
Why is this happening?
I got the idea for this study by reading “Outliers” from Malcom Gladwell. In this book, he names a minor study made in Canada in 1985 about professional hockey players from a team. The conclusion was the same. By more digging into that over the internet, it seems that there’s something called RAE (Relative Age Effect) that sports experts use to study.
What does it take to be along the best players? Without any doubt, there are some basic skills that use to come with each one of us, like talent and intelligence. But, if there’s a common factor along all professional sports people is the training time. This is so necessary that most players start training since a very young age: the more you practice as a kid, the more likely you get to play in first-division.
Here’s a resume I made about what the cannadian study states:

The selection process

What happens when we’re still kids and we play soccer in some team? It’s easy: in order to level us, we get divided by “category”, according to the year we were born. However, this is not exactly egalitarian: if I was born in January of 1989, I’m going to be playing with someone who was born in December, and therefore I’m going to be 11 months older. Who cares about that? How much bigger am I? Now that I’m 25, not much. You can put me side by side with people who were born in January or in December and you won’t guess which is which. That being said, when we’re kids, being 4 years old or being 4 years and 11 months old gets usually noted physically and mentally. I mean: the team of the 5 year old kids is probably going to lose against the team of the 6 year old kids, because physical difference matters. Therefore, by splitting the boys by year of birth, we are giving competitive advantages to those who were born at the beginning of the year. Then, the question changes: why, if around 10 years later this difference tends to disappear, the boys born at the beginning of the year prevail in their advantage to get to play in professional soccer?
That’s a great question. Let’s suppose we take our 6 years old nephew to play a soccer tournament to our closest sport club in the neighbourhood. As he was born in January, it’s possible he’s bigger and plays better (it’s possible doesn’t mean it always have to be like that, but that statistically speaking he has more chances). As he “plays better”, we get all enthusiast about it. We are not the only one: probably, the coach and other people will also see him as a great player, starting to ease the boy more and more into soccer, inviting him to play more often, to train more…If the kid is good in the play field, we as a society are going to make him play more than the rest, so the next year he will be even better and get more advantage. Something that started as a minor age thing starts to raise the quality difference from the rest because of the natural selection process that’s happening. On the other side, if my nephew is one of the smallets kids, probably he will have a hard time by playing, get used of the other players being better than him, and not being stimulated to continue playing. Therefore, when my nephew reaches the age of 20, he’s going to have a lot less training and played matches than their old buddies. That’s how we can explain why the closed to the beginning of the year you were born, the most chances you have to become a professional soccer player.
Can you imagine what would happen if we made the category cut in another way, like in July? Or September? If this theory is true, it means that this change, that could look only administrative, would make hundreds of players get into first division who wouldn’t be able to do it in the way we are making the selection today. It’s crazy, right?
I made the graphics again, this time according to the player position in the field:

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Mediocampistas por mes de nacimiento

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There is 500% more chances you become a goalkeeper by having being born at the beginning of the year. It actually makes sense: who would think of asking the smallest boy to be the goalkeeper? In another positions, this trends tends to stay true, although it wes a little reduced in forwards. Could it be that the size matters more in defenders and middle field players than in forwards? I think it also makes sense. If you don’t, take a look at Messi.

What would I do if I were a soccer club director

Every top team in Argentina is using the same method to find new players: the junior teams. As of today, they all work the same way: by splitting groups by category. I know it’s not the easiest system to build, but if a soccer club started splitting groups by trimester, or at least by semester, specially the youngest teams, that club would have an enormous advantage with respect to the rest of the clubs: it could start recruiting very young players in a much more egalitarian situation, no matter when they were born. Let’s think about what that over-representation means: there are excellent potential soccer players that miss the opportunity of being selected, just because they don’t shine in their category. If I started looking them by organizing my club in another way, I could be able to hunt young talents who aren’t being hunted by the other clubs just because of this “bug” in the classification system.
Would it be so crazy to think in a different pre-selection system?

I found a country that has it. I give you only the graph so you can guess how the system works over there 😉

Bundesliga por mes de nacimiento

Final conclussions

Today, the relative age effect is chosing for us who is good at playing soccer and who isn’t. This distortion is making the system to work in the wrong way. The most we can neutralize this, the more precise we will become by selecting the best players, increasing our players’s quality. As you can see in the graphs, it’s not a minor effect, and focusing in normalizing it could affect for good a lot of people. Do we want to be the best at soccer? Let’s select the real best ones, since they are kids. Let’s take advantage of the power of data to make the right choices. Knowledge makes us responsible.
PS: If you want to have a tennis player son, be careful too with the last trimester of the year!
Tenistas ATP por mes de nacimiento