Steven Levitt writes this interesting post on the impact of age cut-offs and the typical birth-month of players in the NHL. This follows up an article with Stephen Dubner in the NY Times Magazine on the phenomenon in soccer. I’m swamped with grading so I won’t write much about the theory. I’ll just provide you with a quote from the article and two bits of information.
Since youth sports are organized by age bracket, teams inevitably have a cutoff birth date. In the European youth soccer leagues, the cutoff date is Dec. 31. So when a coach is assessing two players in the same age bracket, one who happened to have been born in January and the other in December, the player born in January is likely to be bigger, stronger, more mature. Guess which player the coach is more likely to pick? He may be mistaking maturity for ability, but he is making his selection nonetheless. And once chosen, those January-born players are the ones who, year after year, receive the training, the deliberate practice and the feedback — to say nothing of the accompanying self-esteem — that will turn them into elites.
1) Here is a graph of of the birth-month of every major league baseball player through the 2005 season (16,067 players), thanks to the Lahman Baseball Archive.
(note: the horizontal line at 8.33% is the mean)
2) The cut-off for participation in Little League TM baseball is July 31.
This seems to fit with the theory. Of course, it could be that baseball fans—who are most likely to raise baseball players—have a lower opportunity cost for procreation after the World Series.
Addendum: I re-read the Little League page, and there is a proposal to move the cut-off to April 30. If it passes, in a decade or so, someone remember to check on this. 🙂
Further Addendum: Here is a revised graph of the percent born by birth-month for US-born MLB players. There are four samples for players who made there debuts after 1970, 1980, 1990, 2000. If anyone wishes to play around with the data, it’s freely available at www.baseball1.com.
Then pattern still generally holds; although, the rise in March births is interesting.