The Relative Age Effect in Achievement

Key Insight

A significant, non-random distribution of birth dates exists among elite athletes and high-achievers, known as the 'relative age effect.' This phenomenon was first identified by psychologist Roger Barnsley in Canadian junior hockey in the mid-1980s, where an overwhelming majority of players were born in the early months of the year. For instance, Barnsley found nearly 5.5 times as many Ontario Junior Hockey League players born in January compared to November. Across elite hockey groups, 40 percent of players are born between January and March, 30 percent between April and June, 20 percent between July and September, and only 10 percent between October and December.

This pattern stems from arbitrary eligibility cutoff dates. In Canadian hockey, the cutoff is January 1. Consequently, children born in January, February, or March can be up to 12 months older and significantly more physically mature than peers born later in the same calendar year when selection processes begin around ages nine or ten. Coaches, often mistaking maturity for inherent ability, select these older, larger, and more coordinated children for 'rep squads.' These chosen few then benefit from superior coaching, better teammates, playing 50 to 75 games per season (compared to 20 for others), and two to three times more practice, creating an 'accumulative advantage' that makes them genuinely more skilled by their mid-teens.

The relative age effect extends beyond hockey to other sports and even education. In US baseball, a July 31 cutoff results in more major league players born in August (505 in 2005) than July (313). European soccer leagues, with various cutoff dates like September 1 in England, exhibit similar skewed birth-date distributions. In education, economists Kelly Bedard and Elizabeth Dhuey found that among fourth graders, the oldest children scored 4 to 12 percentile points higher on math and science tests, a 'huge effect' impacting gifted program qualification. This initial disadvantage persists, with the relatively youngest students in US four-year colleges underrepresented by approximately 11.6 percent, highlighting how early, arbitrary distinctions based on birth date can profoundly shape long-term achievement.