How much of a difference does it make when your birthday is?

Consider, for instance, those hockey stars. Relying on the work of a Canadian psychologist who noticed that a disproportionate number of elite hockey players in his country were born in the first half of the year, Gladwell explains what academics call the relative-age effect, by which an initial advantage attributable to age gets turned into a more profound advantage over time. Because Canada’s eligibility cutoff for junior hockey is January 1, Gladwell writes, “a boy who turns 10 on January 2, then, could be playing alongside someone who doesn’t turn 10 until the end of the year.” You can guess at that age, when the differences in physical maturity are so great, which one of those kids is going to make the league all-star team. Once on that all-star team, the January 2 kid starts practicing more, getting better coaching, and playing against tougher competition—so much so that by the time he’s, say, 14, he’s not just older than the kid with the December 30 birthday, he’s better. The solution? Double the number of junior hockey leagues—some for kids born in the first half of the year, others for kids born in the second half. Or, to apply the principle to something a bit more consequential (to non-Canadians, at least), Gladwell suggests that elementary and middle schools put students with January through April birthdays in one class, the May through August birthdays in another, and those with September through December in a third, in order “to level the playing field for those who—through no fault of their own—have been dealt a big disadvantage.”

(Excerpted from New York Magazine)