May 13, 2006

Birthday bias in the the National Hockey League: Nature vs. Nurture

World Cup soccer players (typically the top 20 or so players from each of the top soccer countries) are born in equal amounts in the first half and last half of the year (although misreported in the New York Times by guess who?). Yet, there is a birthday bias among NHL hockey players, currently running at 59-41 for those born in the first half of the year compared to the second half.

Let me try to sum up the implications of the hockey example and relate them to Dr. Levitt’s ambitious statements about the relative importance of nature and nurture.

To be drafted by the NHL at age 18, a Canadian youth must pass through an ever narrowing funnel of selection. In particular, he must distinguish himself in youth hockey competitions at the national and world levels that are restricted to 17-and-under players. This gives an advantage to those 17-year-olds who are almost 18 compared to those 17-year-olds who have just turned 17.

Let’s assume for the moment that this 59-41 difference in first half versus last half of the year birthdates in the NHL reflects a genuine difference in mature performance level rather than a market inefficiency, and that it stems solely from the early-born people getting better nurture than the late people.

So, does this fact settle the nature vs. nurture debate inaugurated so long ago by Sir Francis Galton?

Well, the nature glass is part full and part empty, just as the nurture glass is part full and part empty. But, what are the proportions?

Very roughly speaking, one in every ten thousand Canadian males between 18 and 40 is playing in the NHL.

One factor influencing who gets into the NHL appears to be the luck of the birth date. Somebody born in January is about 1.7 or so times more likely to make the NHL than somebody born in December. So, the odds for somebody with the good luck to be born early in the year might be 1/7,500 versus 1/12,500 for somebody born late in the year. (These are just back-of-envelope estimates of relative magnitude.)

So, that is a significant role for nurture, but not an overwhelming one, since in a national sport like hockey in Canada subtle opportunity effects matter mostly to the far right edge of the bell curve for athleticism.

I'm sure there are a huge number of other nurture factors like quality of coaching, parental fanaticism, and so forth. But, let's take a rough swing at estimating the magnitude of nature and nurture in the chances of a Canadian making it to the NHL.

I think it’s safe to say that nobody in the NHL is less than one standard deviation above the mean in natural hockey athleticism, which eliminates 84% of the population. The best training in the world will never make a mediocre or below average athlete into an NHL player.

Further, I would guess that almost nobody in the NHL is less than two standard deviations above the mean (although I could be wrong), so that would be 97.7% of the population that doesn’t have a chance.

Among the remaining 2.3%, however, I would imagine that nurture is highly important.

This is not to say somebody at the 99.9999th percentile in natural talent has no better chance than somebody at the 97.3rd percentile. For example, here is Wikipedia's profile of the early years of The Great One, Wayne Gretzky. Although Gretzky was born January 26th, that made little difference in his youth career since he constantly played against older athletes.

Taught by his father Walter, Gretzky was a classic prodigy. At age 6 he was skating with 10-year-olds. At age 10 he scored 378 goals and 139 assists in 85 games, and the first story on him was published in the Toronto Telegram. At 14, playing against 20-year-olds, he left Brantford to further his career and signed with his first agent.

He played a season in the Ontario Hockey League at the age of 16 with the Sault Ste. Marie Greyhounds...

He became the youngest player to compete in the World Junior Championships, when he participated in Montreal in 1978 at age 16. Despite being the youngest player in the tournament by far, he finished as the tournament's top scorer, was voted to the All-Star team and Best Forward of the tournament.

That year (1978-79) he signed with the Indianapolis Racers of the World Hockey Association (WHA) as an underaged player. The National Hockey League (NHL) does not allow the signing of players under the age of 18, but the WHA had no rules regarding such signings. Racers owner Nelson Skalbania signed the 17-year-old to a personal contract worth between 1.12 and 1.75 million dollars US over 1 to 2 years.

While I was living in Houston in 1979 or 1980, a college roommate told me that everybody in Canada knew that this teenager named Wayne Gretzky was going to be the the greatest hockey player of all time, and the only question was whether he was already the great player.

Keep in mind, though, that ice hockey in Canada is of course one of the most competitive selection environments in the world. In less popular sports, however, flukes of environment matter far more. The chance of an American kid making it to the NHL is much more driven by things like geography (e.g., a Minnesotan is a lot more likely to make it than a South Carolinian).

To take an extreme example of the dominance of nurture over nature, in the 1970s an American college student read that Team Handball would be an official sport at the 1976 Olympics. So, he convinced his fraternity brothers to take up the game and practice it for a few years. The fraternity qualified en masse to represent America in Team Handball in Montreal, and presumably had a blast (at least while they weren’t getting thrashed on the court by countries that actually cared about the sport).

The mean natural athleticism of those fraternity brothers was probably only slightly above average, but in the utterly non-competitive environment of Team Handball in the USA 30 years ago, they were able to leverage their nurture advantage to become the best in America.

So, this comparison reflects a general principle that the the more meritocratic and competitive a competition becomes, the more nature outweighs nurture.

My published articles are archived at -- Steve Sailer

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