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Born on the 4th of July August?

Posted by Doug on October 23, 2006

Let me regale you with tales of my childhood glory on the baseball diamond.

When I was growing up, I played each year in a spring league and a summer league. The spring league was organized by grade (the age cutoff was Sept 1). The summer league was organized by age, with the age cutoff being Aug 1. I was born in late August, which meant that I was always among the youngest people in my grade, but among the oldest in my summer league baseball age group.

So, for example, when I was 14, I was in 9th grade. In the spring I played with 9th- and 10th-graders (the high school JV team), because I was in 9th grade. But in the summer, I could play in two leagues. I played with the 9th- and 10th-graders because that's who I had played with in the spring (and because the summer league allowed for playing up). But I also played with the 7th- and 8th-graders because I was only 14.

I believe that setup was the best of all possible worlds for my baseball development. It gave me the chance to get accustomed to "the big time" while also allowing me to be a star in the younger league. Granted, I'm teaching math and writing a blog instead of playing professional baseball, but I was a pretty good ballplayer back in the day, and I've always believed that my August birthday had a lot to do with it.

My buddy JC Bradbury posted some data over at sabernomics that show that I'm not alone. More major league ballplayers were born in August than in any other month. But the data show more than that. August/September/October birthdays are much more common than May/June/July birthdays, which indicates that it was probably the playing down that helped me more than the playing up.

The idea is summarized in this passage from Steven Dubner, whose work (with Steven Levitt) is what inspired Bradbury to look into it:

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.

Evidently the same phenomenon is observed, to some extent, in soccer and hockey. This NYT article examines whether or not it might play a role in academics as well.

So I decided to check out the football situation. Among all players currently playing in the NFL (except for offensive linemen, who are not in my database), the birthmonths break down like this:

Month #born
1 119
2 111
3 126
4 88
5 91
6 94
7 92
8 129
9 118
10 98
11 113
12 105

The pattern is not quite as clear as it is in baseball (possibly because I have a lot less data), but it nonetheless appears that May/June/July is a bad time to be born if you have aspirations of being a professional athlete. Of course, all this assumes that birthdays are spread equally throughout the year for the general population, which is probably not correct. Still, 40% more current NFL players were born in August than in July, and I seriously doubt that the difference is that dramatic in general.