Age Cut-Offs and Month of Birth in Baseball

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.

Birth-month of MLB players
(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

Birth-month of  US-born MLB players

Then pattern still generally holds; although, the rise in March births is interesting.

11 Responses “Age Cut-Offs and Month of Birth in Baseball”

  1. David Kane says:

    Could you show us other versions of this graph?

    1) How about US-born versus non-US-born? If there is some Little League age effect, one would expect very different patterns in the two groups (assuming that July 31 is not also the cut-off in Mexico, Cuba and so on).

    2) How about showing only current major league players versus pre-1970? Although the July 31 Little League effect might be a big thing now (and one or two decades ago when current players were younger), I would predict that it would be less true, say, before 1970. Who knows what the magic cut-off date was in 1955? Also, the number of special elite teams and the like was much less 40 years ago.

  2. shonk says:

    For players currently in MLB, the more important cutoff is probably that for AABC and other competitive baseball leagues, not Little League. Of course, since they all follow Little League’s lead, the effect is the same. Another important consideration is probably the cutoff dates for school grade-levels (typically sometime between Sept. 1 and Oct. 1). As a 13 year-old freshman, I remember the frustration of competing for playing time against 15 year-old freshmen, especially given that differences in physical maturity are so pronounced at that age (of course, my case was somewhat extreme since I skipped a grade in elementary school).

  3. studes says:

    I believe this closely follows the normal pattern of births by month in general. Anyone who’s ever worked in a hospital knows that September and October are high months for births.

  4. JC says:

    Dave Kane,

    See Addendum.


    You’re right. The school cut-off ought to be important. The spike in October is interesting.


    I didn’t look up the normal pattern of births, but Levitt posted this info on his page and it looks a bit different than the pattern in baseball.

  5. studes says:

    A bit different, but it doesn’t appear to be wildly different (at least to my eye). The one difference that does jump out (again, to my eye) is the July/August difference. I think that would confirm your hypothesis, in a very “localized” sense (seems to impact a couple of months quite a bit, but the effect is less pronounced — if it is significant at all –in other months).

  6. David Kane says:

    The Levitt debate got interesting later in the link that you cited. Turns out that his data was from an astrology site! Not that there is anything wrong with astrology, of course.

    Seems to be something going on in hockey, but not among the better players. See the thread for details. It appears to be a draft inefficiency.

    So, the hypothesis for baseball would be that there might be a similar inefficiency (people are too quick to draft older high schoolers who look better than they actually are). Prediction would be that if you redid this graph looking only at, say, all-stars, the effect would go away.

  7. Brian Hammond says:

    FYI – Little League actually went through with the proposed change to the cutoff date, so it’s now April 30.

    I have multiple reasons to complain – I was born July 26, and was always the youngest guy in LL, while my brother was born Aug. 4. And to add insult to injury, my son’s birthday is April 30. It’s almost enough to drive someone to put their kid in soccer. OK, no it isn’t.

  8. Sean Forman says:

    Woo Hoo! Carl Larson Forman born August 5.

  9. Dan Green says:

    My baby son will be born in the next 10 days. I was depressed, and then I saw that the April 30 debate is happening.

    Time to call my lobbying friends in Williamsport.

  10. Bruce Meyers says:

    When my younger son was born on August 1 at 2 am I immediately knew the significance. My wife, of course, was not in the mood to care much.

    But that extra year was important in LL for him. He was small for his age, but the “year” was very helpful. When selecting players for teams I looked at the birthdates as a factor.

    While not with LL per se, in HS sports, those children who stayed back a year along the line do have an advantage as well. I wonder how much this affects which ones, because of being a year older, make it on athletic scholarships for college.

    Enjoy the website very much.


  1. [...] ielder when I was 12. I never made it. Now I know why — I was screwed. Screwed by my month of birth. Who do I sue?