From FiveThirtyEight:
FEB. 22, 2021, AT 10:01 AM
Why Athletes’ Birthdays Affect Who Goes Pro — And Who Becomes A Star
By Tim WigmoreIf you want to be a professional athlete in most sports, it helps to be born at the right time of year.
In basketball, baseball and ice hockey, players born in the first quarter of their selection year — the cutoff for which age-group teams are picked, which is normally the school year — are overrepresented both in youth and professional sports. In soccer, players born in the first quarter of their selection year are overrepresented throughout major leagues in Europe and South America.
This phenomenon, called the relative age effect, impacts almost every sport. It has been demonstrated in both men’s and women’s sports, although the effect seems to be less pronounced in women’s sports.
Of course, children old for their cohort don’t have better genes or more talent. Instead, “the relative age effect is almost certainly related to differences in rates of biological and psychological maturation,” said Joe Baker, a professor at York University in Toronto who researches the relative age effect. “Those who are relatively older appear stronger, faster, etc., but they’re really just older and therefore more advanced in their maturation.”
Such children are more likely to be picked for school teams. Once they are picked, players benefit from more practice, coaching and game time — advantages denied to those not selected, who are disproportionately likely to be younger for their selection year. Once accounting for their biological age, the older players might not have been any better than later-born children when they are first picked. But after becoming part of a team, and being exposed to training and matches, they really do become better than later-born children who might be equally talented.
One clear trend holds true across sports: The younger that talent is identified, the greater the relative age effect. This makes intuitive sense; on a fourth grade sports team, made up of 9- and 10-year-olds, the oldest child could be 10 percent older than the youngest.
This all suggests a way of countering the relative age effect: to delay selection for as long as possible — until mid-adolescence or even later. In practice, Baker said, “coaches could work to ‘soften the blow’ of not being selected,” by keeping late-maturing players in the system for longer, thereby giving them the best possible chance of fulfilling their potential later. New Zealand’s youth rugby teams have long grouped young players together based on weight rather than age; the method means that skillful but smaller players are not disadvantaged. Similarly, in soccer, teams have tried methods including “bio-banding” — grouping together players based on their height and weight, not age. Teams have even explicitly arranged trials for those born in the last months of the selection year in case their talent has been missed.
At every turn, school sports are effectively rigged for children born soon after the school start cutoff date — which varies by state in the U.S. but is most commonly around Sept. 1 — and against later-born children. Yet in the NBA, James Harden1 and Kobe Bryant2 are among the former MVPs born in August, putting them among the youngest in their school year group. In baseball, Mike Trout and Cody Bellinger were each just 17 years old when they were drafted; Tom Brady and Barry Sanders are among the NFL MVPs who were young for their cohorts.
Mike Trout was born August 7, 1991, so he graduated high school at age 17 in 2009, and then was drafted 25th overall. If he’d been born September 7, 1991, he would have graduated at age 18 in 2010. Presumably, he would have hit about .600 in another year of high school ball and been the #1 draft pick that year, even over Bryce Harper.
Tom Brady was born August 3, 1977, so he graduated from high school at 17. He was quite good in high school, being recruited by schools that traditionally have strong quarterbacks like UCLA, Cal, and Michigan. He then redshirted his first year at U. of Michigan and then played four more seasons. But he was always rather youthful-seeming. He didn’t wow the NFL as a draft prospect and was the 7th QB drafted that season. But youthfulness is a good thing in an adult athlete, so he played through age 44.
Kobe Bryant was born August 23, so he was 17 when drafted 13th by Jerry West of the Lakers.
These athletes embody a notable paradox: Once they reach professional levels, younger-born players tend to be more successful and are overrepresented among “super-elite” athletes. This phenomenon, found across a range of sports and explored in “The Best: How Elite Athletes Are Made,” a book I co-authored, is known as the underdog effect. Essentially, it is harder for later-born children to become professional athletes — but if they do become professional, they have a higher chance of becoming among the very best players in their sport.
A study of the most valuable male players in professional soccer, ice hockey, baseball and Australian rules football analyzed their birth dates relative to the selection year for the sport in their country. The finding was the opposite of the relative age effect: Players born later for their year were overrepresented among the very elite, accounting for a combined 55 percent of players.
One interesting question I haven’t seen addressed is whether kids switch out of sports where they have a relative age disadvantage to one in which their birth date is more auspicious. A test case might Canadian athlete Larry Walker, who grew up focused on being a hockey goalie, but when he washed out at 19 he switched to baseball and went to the Hall of Fame. I have to imagine Walker had the skill set to be a hockey goalie. But he was born on December 1, 1966, so he was always eleven months younger than his oldest rival born in 1966, which likely affected his trajectory through youth hockey and kept him from getting the best coaching.
