I analyzed the 736 players in this years world cup: ~30% are born in the first quarter, while only ~20% are born in Q4.
So why is this so?
Here’s what author Malcolm Gladwell said to ESPN (about similar skewness among Canadian hockey-players):
It’s a beautiful example of a self-fulfilling prophecy. In Canada, the eligibility cutoff for age-class hockey programs is Jan. 1. Canada also takes hockey really seriously, so coaches start streaming the best hockey players into elite programs, where they practice more and play more games and get better coaching, as early as 8 or 9. But who tends to be the “best” player at age 8 or 8? The oldest, of course — the kids born nearest the cut-off date, who can be as much as almost a year older than kids born at the other end of the cut-off date. When you are 8 years old, 10 or 11 extra months of maturity means a lot.
So those kids get special attention. That’s why there are more players in the NHL born in January and February and March than any other months. You see the same pattern, to an even more extreme degree, in soccer in Europe and baseball here in the U.S. It’s one of those bizarre, little-remarked-upon facts of professional sports. They’re biased against kids with the wrong birthday.