The birthday effect

Pick any youth team and the oldest kids on it are usually the best. At eight, a child born in January has lived a ninth longer than one born the following December. Coaches pick the bigger, quicker kids for the better teams, those kids get the better coaching, and the gap grows. Researchers call it the relative age effect. We checked the birthday of every player in five sports to see how far it reaches.

It reaches all the way to the top in some sports. In hockey, for every player born in the last three months of the year there are 1.6 born in the first three.

The stars are spread more evenly

In hockey, the effect is much weaker among the best-known players than among everyone: 1.12 for the most famous tenth, against 1.59 for all of them. Researchers have found the same thing elsewhere. The late-born players who do make it have had to be better to get there, so they’re more likely to become stars once they arrive. Sidney Crosby was born in August, Brett Hull in August, and Chris Pronger in October. Soccer is the exception in our numbers: its best known lean toward early birthdays as much as everyone else does.

Why it matters

Every hockey player born in the last months of a year who never made it is a player the sport lost to the calendar, not to talent. Some youth programs now try to fix it, with birth-quarter teams or by rotating the cutoff. For the rest of us it’s a good tiebreaker: in Teammate Chain or any hockey trivia, a player you can’t place was probably born in the first half of the year. And see when careers peak for what happens at the other end of a career.

How we counted

Every player who appeared in at least one regular-season game in our records and has a known birth date counts once. A month’s figure is its share of the players against its share of the days in a year (February counts as 28¼ days), so 100 is exactly even and +25% means a quarter more players than an even spread would give. Births aren’t perfectly even through the year in the general population either: in the United States and Canada, late summer and early fall are a few percent busier than spring. So a month within about 10% of even says little; the patterns that matter here are much bigger. The best-known tenth is chosen by the same fame score the games use. Soccer birth dates known only to the year are recorded as January 1, so those 234 players are left out.

About the data