Why many Olympic athletes have early birthdays | Science News for Students

Why many Olympic athletes have early birthdays

Older kids in any year tend to be picked first for teams, giving them edge in training and competing
Feb 20, 2018 — 6:45 am EST
Andrew Weibrecht

Andrew Weibrecht races downhill at the 2010 Winter Olympics in Vancouver, Canada. He turned 32 at the 2018 Winter Olympics on February 10.

Kevin Pedraja/Wikimedia Commons (CC BY 2.0)

In a curious coincidence, two members of the U.S. men’s Olympic alpine ski team will celebrate their birthdays during the Winter Games in PyeongChang, South Korea. Another five of them will be lighting birthday candles in the two months surrounding the event.

This could be random chance. Or not. The way that coaches pick teams when kids are very young could give an advantage to those born at a certain time of year. Slightly older kids will be just a bit bigger than younger teammates born the same year. In many sports, that difference is a bonus. But it means coaches might tend to leave younger kids on the bench.

Steven Nyman
Andrew Weibrecht didn’t celebrate his birthday alone. Teammate Steven Nyman turned 36 on February 12, while competing in the 2018 Winter Olympics.
Kevin Pedraja/Wikimedia Commons (CC BY 2.0)

This can give older kids a small advantage in sports. And what starts in childhood may continue on throughout their sports career. No one doubts that professional athletes work hard and are extremely talented. But in sports like hockey, skiing and soccer, many players tend to be born around the same time of year. This early birthday boost is known in science as the “relative age effect.”

Late bloomers shouldn’t despair, though. Given some time to grow, later-birthday kids might equal — and even surpass — older teammates. 

Birthday bias

Older kids get a boost both on and off the field, says Nick Wattie. He studies the science of sports in Canada at the University of Ontario Institute of Technology in Oshawa. Older kids often get better grades, for instance, and are more likely to be identified for gifted programs.

But the effect is easiest to spot in sports. Early-birthday athletes get professional careers in sports more often in swimming, soccer and hockey, studies show. “Older athletes may look more talented because they are faster, stronger,” says Joe Baker. He studies sport science at York University in Ontario.

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birthday graph
Out of 13 men on the U.S. alpine ski team competing in the 2018 Winter Olympics, seven have January to March birthdays. This is an example of the “relative age effect.”
B. Brookshire/SSP

Not every sport favors the slightly older kids. For example, American football doesn’t have a strong birthday bias, Wattie says. “In youth football, you’ve got age and height and weight categories,” he notes. “So even if you’re smaller, you still have a chance to play and have a good experience.” That good experience means younger football players might stick with it — and their talents may be eventually spotted — as they grow older.

kid gymnast
In sports like gymnastics, being a bit younger might hold benefits. Young kids tend to be more flexible.

Other sports show no age advantage because they don’t rely on the same types of speed or size. In gymnastics and figure skating, for example, younger kids actually may have the advantage because they can be more flexible. And shorter kids may be better suited to the leaps, turns and flips that wow the crowd to win the gold in these sports, says Baker.

Gender also may play a role, Wattie notes. Men’s sports seem to show a birthday bias more than women’s. Perhaps this is because girls finish growing sooner than boys. “So by the time you’re 13 to 16, and competition begins to heat up among future pro athletes, things have balanced out,” Wattie suspects.

Growing out of it

Boys can catch up, too, once they reach adult size, notes James Salter. He is a coach with Swimming Australia. This group governs competitive swimming in Australia. (Salter also competed for Great Britain in swimming in the 1996 and 2000 Summer Olympics.)

In swimming, size matters. So age also matters. The slightly longer arms and legs of somewhat older kids helps them swim farther and faster with less energy. But Salter and his colleagues have found that this effect exists only for boys between 12 and 15 and for girls between the ages of 12 and 14. By 16, the younger kids have grown up — and caught up in the pool. These findings appeared December 29 in the Journal of Science and Medicine in Sport.

“Among the youngest kids, many won’t manage to make it,” says Luca Fumarco. He studies economics at the National Institute of Statistics and Economic Studies in Luxembourg. But if those younger, smaller players stick with it, talent may eventually trump age.

In the North American National Hockey League, for instance, more players are born in the spring, Fumarco and his colleagues find. But players with later birth dates who did make it into the pros scored more points. They also made more money — as much as 51 percent more than their older teammates.

Fumarco and his colleagues published their findings August 14 in PLOS ONE.

“Some of them, because they are naturally more skilled, they manage to be selected or to find more time on the ice,” Fumarco notes.

Science for the underdog

Coaches want to find the most talented athletes for their teams. They don’t want to make a mistake and leave a potential world record holder sitting on the bench. But fighting birthday bias isn’t easy. Should coaches divide up teams into smaller and smaller age groups? Should they sort everyone by weight, like youth football teams?

soccer kids
Indicating age-order on a child’s shirt might help scouts pick recruits based on talent alone, a study suggests.

“There have been a bunch of suggested solutions put forward but no evidence to show that any of them work,” says David Mann. He specializes in sport science at Vrije University Amsterdam in the Netherlands.

Mann tested one potential solution on the field. He filmed 10-year-olds playing four-on-four soccer. The kids all wore numbered shirts. Each number denoted the player’s age relative to the rest of the team. Player one was the oldest on the field, player two the second oldest. Player eight was the youngest.

He then showed the video to 25 soccer talent scouts. He asked them to pick the most talented kids from the field of eight. (Don’t worry, it wasn’t real. No one was picked for a soccer team, or booted off, for science.) Mann instructed one group of scouts simply to identify the best players. He gave another group a list of the players, with birthdates. The others were told what those numbers on the kids’ shirts meant.

The first group of scouts chose the oldest kids. Scouts with a list of names and birthdays also tended to select older kids. But when scouts knew the meaning behind the numbers on the players’ backs, their behavior changed. The preference for the oldest players disappeared. Mann and his colleagues published their finding two years ago in the Journal of Sports Sciences.

Age bias may be worth fighting. “My job is to provide these athletes with the opportunity to be successful,” says Salter, the swimming coach. To do that, he must ensure he does not overlook kids just because of their age.

It’s also important, he notes, to keep this bias in mind so that slightly younger kids don’t get ignored or sidelined — and quit — before their skills shine. “We have to provide different opportunities so they have a positive experience in the sport,” he says. “No one wants to [compete] if they’re not enjoying it.”

Age is “not a seal on someone’s fate,” though, says Wattie. Other factors such as access to sports equipment, teams, coaches and supportive parents can play a more powerful role. And the biggest difference of all? Practice. 

Power Words

(for more about Power Words, click here)

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

bias     The tendency to hold a particular perspective or preference that favors some thing, some group or some choice. Scientists often “blind” subjects to the details of a test (don’t tell them what it is) so that their biases will not affect the results.

colleague     Someone who works with another; a co-worker or team member.

economics     The social science that deals with the production, distribution and consumption of goods and services and with the theory and management of economies or economic systems. A person who studies economics is an economist.

gender     The attitudes, feelings, and behaviors that a given culture associates with a person’s biological sex. Behavior that is compatible with cultural expectations is referred to as being the norm. Behaviors that are incompatible with these expectations are described as non-conforming.

Great Britain     The collective name for England, Scotland, Wales and their associated islands. This is not the same as the United Kingdom, which is the combination of Great Britain and Northern Ireland.

journal     (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.

mature     (adj.) Connoting an adult individual or full-grown and fully developed (non-juvenile) form of something. (verb) To develop toward — or into — a more complex and  full-grown form of some individual, be it a plant, animal or microbe.

peer     (noun) Someone who is an equal, based on age, education, status, training or some other features. (verb) To look into something, searching for details.

trump     In certain card games, it’s a card that owing to some special feature (such as its suit) allows it to rank above all others. In pinochle, for instance, a 2 of the trump suit can capture even an ace of any other suit. (outside card games) Some feature or action that outperforms or tops all others.


Journal: S. Cobley et al. Transient relative age effects across annual age groups in national level Australian Swimming. Journal of Science and Medicine in Sport. Published online December 29, 2017. doi: 10.1016/j.jsams.2017.12.008.

Journal: L. Fumarco et al. The relative age effect reversal among the National Hockey League elite. PLOS ONE. Vol. 12, published online August 14, 2017. doi: 10.1371/journal.pone.0182827.

Journal: D. L. Mann. Age-ordered shirt numbering reduces the selection bias associated with the relative age effect. Journal of Sports Sciences. Vol. 35, published online May 30, 2016. doi: 10.1080/02640414.2016.1189588.

Journal: N. Wattie, J. Schorer, J. Baker. The relative age effect in sport: A developmental systems model. Sports Medicine. Vol. 45, published online August 24, 2014. doi: https://doi.org/10.1007/s40279-014-0248-9.

Journal: W. F. Helsen. The relative age effect in European professional soccer: Did ten years of research make any difference? Journal of Sports Sciences. Vol. 30, published online September 24, 2012. doi: 10.1080/17461391.2012.671369.

Journal: J. Baker et al. Variations in relative age effects in individual sports: Skiing, figure skating and gymnastics. European Journal of Sport Science. Vol.13, published online April 23, 2012. doi: 10.1080/17461391.2012.671369.