Distribution of Birthdays and Student Performance

I accidentally happened to chance upon the part of Malcolm Gladwell's book "Outliers", where he talks about how the selection system for junior ice-hockey leagues in Canada strongly favors older kids in a particular cohort, and how that effect lingers for a while.

Here's Gladwell describing the idea in an ESPN interview.

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.

The data on that site, and many others such as this one, seem to bear it out. Of course, reality may be more complicated but there does seem to be a germ of truth in the Gladwell's simplified narrative.

Here (pdf) is another study looking at reading skills as a function of birthday distribution, where a similar effect is found.

Why should you care?

Suppose, like me, you have a child whose birthday falls in that weird window (due to arbitrary school cut-off ages), where he or she can either be the oldest or youngest person in her class (depending on your decision to wait an additional year or enroll right away).

I haven't converged on an answer yet, but I know years from now, regardless of what I do, it will all be my fault!