Are stars born or made? Here’s what World Cup 2006 has to say on the issue.

Last month we wrote a New York Times column asking whether superstars are born or whether they are made through a combination of the lots of practice, the right kind of practice, and coaching. The experts in the area suggest that superstars are made.

One conjecture we made in that column was that because the FIFA cutoff date for determining a child’s age for international play is January 1, we would expect that a disproportionate number of the players in the World Cup would be born in the early part of the year. The idea is that these kids will get special treatment and attention when they are teenagers because they will be developmentally almost a full year ahead of kids born in the latter part of the year. These developmental differences will fade away with age (i.e. by the time you are 25 it doesn’t matter much if you are 25 years and 1 month old or 24 years and two months old), but the early success, access to coaching, playing experience that the older kids got as teens would have long term effects on their soccer careers.

This conjecture led to some vigorous blog commenting (see here and here).

There were two nuances we missed though. First, the January 1 eligibility date wasn’t put into place by FIFA until 1997. And also, individual countries use different month cutoffs for determining who is eligible for different leagues within the country. Many countries use a date in the fall for their cutoffs, but there are a whole range of eligibility dates.

So, the clearest prediction we are left with concerning the World Cup is that if the FIFA January 1 cutoff matters and the experts in this area are correct, then for players born late enough to be affected by the 1997 date change should be disproportionately born in the early months of the year, and much less likely to be born in the last few months of the year. For players who are too old to be affected by the 1997 FIFA rule change (i.e. they were not teenagers at the time of the change), it is unclear what the pattern will be, but there is certainly no reason to expect a lot of January players. To really do these older players right, one would have to go country by country to determine what the cutoff rules were. (Something we haven’t done, but which I suppose some World Cup crazed blog readers will decide to do.)

We found a spreadsheet with the dates of birth of all players on World Cup rosters in 2006. What do the data say?

First, among players born in 1979 or later, making them 18 or younger when the FIFA January 1 cutoff took effect, the months of birth look like this:

January — 52
February — 43
March — 35
April — 38
May — 38
June — 25
July — 29
August — 31
September — 26
October — 35
November — 22
December –27

32.4 percent of the players were born in the first three months of the year, 25.2 percent of the players were born in months four through six, 21.5 percent in july to september, 21 percent in the last three months. Exactly what the theory predicts.

How about for the older World Cup players, those born before 1979? A very different pattern emerges:

January — 22
February — 23
March — 22
April — 23
May — 27
June — 31
July — 29
August — 36
September — 34
October — 27
November — 32
December –29

For the older players, only 20 percent of the older players were born January to March. 24 percent were born April-June. The months July-September were the most common.

All in all, this seems like pretty strong evidence in favor of the theory.