Maybe the World Cup Wasn’t the Best Example

In our recent New York Times column, we talked about what makes people good at what they do.

As one example, we conjectured (based on some academic work done by others) that people born in the early months of the year would be overrepresented on World Cup rosters. The underlying theory is that in 1997, FIFA made January 1 the cutoff for determining ages in all international soccer competitions. If this rule had an important impact in determining who made the national youth soccer teams, then these early selection rules would play out to more long run success at the highest levels of soccer. The academic evidence is that these national teams are overwhelmingly made up of players born early in the calendar year, even on the age 21 and under teams, where a few months of physical development isn’t likely to make a big difference. A commenter on our blog, Bill Loyd, has done some hard work to gather data and argues that for past World Cups and for a few of the 2006 squads that he found, he doesn’t see the pattern we predict.

Why might this be the case? For the earlier World Cups, it might not be very surprising that no pattern is there because the FIFA rule didn’t come in until 1997. More fundamentally, the FIFA selection rules and the rules that different countries use for play within the county differ.

For instance, as many readers have emailed us, in the U.S., the age cutoffs tend to be in the summer. In Germany, the within country age cutoff is August 1. Thus, in soccer there are two different competitive pressures at work: one pushing towards more players born in the early months and the other towards more players in the later months. Much of the study of birth-date timing focuses on the cutoff rules within countries, virtually all of them finding important effects.

In light of this difference between FIFA and country rules, the example we gave of the World Cup might not have been the best one, even though the age effect is very strong in the national youth squads that feed many World Cup teams.

This shouldn’t distract from the important fact that the evidence in the literature overwhelming supports the basic point — that across many activities, you can identify long-term effects of essentially arbitrary age cutoffs early in life.

Perhaps a better example than the World Cup would have been the N.H.L. Here is one graph that I found on the web of the birth month of NHL hockey players versus Canadians and Americans more generally:

The black-and-white dots are the NHL players, who are much more likely to be born in January and February and much less likely to be born September-December. This is the sort of pattern that appears over and over in these sorts of studies.

Some other readers have offered a clever, very Freakonomics-y alternative explanation for these age patterns: the parents are lying about their child’s birthday. If the parents want the kid to be a star, they take an older kid and change his date of birth to make him eligible to play with younger children. While I don’t think this is actually the primary reason for what people find in these studies, is definitely worth thinking about.


This is a general comment about another freak-o change to a formerly exclusive and greedy "profession."

The article:
Title: "The Secret Source is Out"

This article documents that the web and other pressures are changing the privileges of interior decorators. They used to get outrageous commissions partly because they had sources of products open only to "the trade." Now more and more such sources are capitulating to pressure to sell to any buyer. Sound like realtors? Sound like travel?


I used to see the same age effect as a swim coach. For our summer club, the age cutoff was June 1; so, calendar-age 15 year-olds competing against 13-14 year-olds, and they tended to fare substantially better.

However, I'm curious why you didn't consider the more obvious age-cutoff example: school enrollment? As an October birthday, besides being among the last to drive, I also always thought I was among the least socially adept, whereas my Nov and Dec birthday friends, who were one-grade below me in school, seemed to fare better. Of course, I could be fishing for an excuse for myself, but...

Also, I've always thought that similar age cutoffs happen with childbirth, that you should use gestational age (assuming you can accuratley measure the conception date) rather than calendar age for lots of early childhood milestones, that it is better (for the child) to be in utero for 40 weeks than 36 weeks, except to the degree that endangering the mother's help endanger's the child's. For many milestones, it probably doesn't matter too much because they get the same exposures out of the womb, but I would think that the extra time would in some way better position the child physically/developmentally to handle what they're exposed to.



rsaunders, The papers Levitt and Dubner link to do show that a late month born child is more likely to be identified as "learning disabled," be retained, etc.
I'm only guessing the authors chose the more sexy sports data over the most disconcerting, depressing, or alarmist data for their article.


Seems to work for baseball too.


"one graph that I found on the web" is an, uh, interesting piece of evidence. Amazing what appears on the web. Why not provide a link so that Bill Lloyd can check if this claim is as, uh, accurate as the previous claims made about World Cup soccer players?

Stephen J. Dubner

A reader writes:


I am a hockey broadcaster in Canada who thoroughly enjoyed "Freakonomics".
I read with interest the piece in the NY Times magazine about European
soccer stars and their birth months ("A Star Is Made", May 7).

Hockey provides an interesting case study. Most elite hockey players from
North America are born in the first three months of the year. In fact,
approximately 3/4 of the players in the Hockey Hall of Fame were born in
January, February or March.

But it's an entirely different case among elite level European players,
especially those from the old Eastern Bloc. Why? Because unlike North
America, hockey programs in those nations are part of the school system, and
thus use the school calendar as their basis. When looking at the highest
scoring European players, we find mostly September, October and November

The "birth month" issue is well known in Canadian hockey circles, as
evidenced by an episode in suburban Toronto several years ago. A couple had
a child born on New Year's eve, very close to midnight. The father begged
the doctor to alter the birth records to say that the child was born on
January 1.

The doctor suspected that the father may have been trying to pull a "New
Year's baby" scam and refused. The discussion became heated, and the police
were brought in.

It turns out the father was an obsessed hockey parent who wanted to give his
son "a leg up" in his future hockey endeavors by having his child be the
oldest in his age group as opposed to the youngest.

Your work is entertaining and excellent. Please keep it up.


Gord Miller
The Sports Network


Stephen J. Dubner

To dkane:

I don't know where Levitt's graph comes from, but here's a link we've cited a few times in related threads here that should be of interest to you:


I'm glad you guys posted this correction/follow-up! Interesting topic. I skipped a grade and have a late birthday, so I'm pleased to have an excuse for my poor athletic skills.

David Kane

Mr. Dubner,

Thanks for taking the time to provide that link. No doubt I am an idiot, but after looking at the site for 15 minutes, I could not find a single piece of data that supported the factual claim that --- at the adult, elite level (i.e., World Cup soccer, NHL hockey) --- there is an month-of-birth effect. (Everyone seems to agree that there is such an effect under age 20 or so.)

Could you specify the precise location at that site with such data?

Thanks for your time.

Bill L. Lloyd

Mr. Levitt,

Thank you for posting on my World Cup findings.

I'm afraid I still don't see any good evidence for your and Mr. Dubner's thesis.

Not trusting a graph that you "found on the web," with no citation, I tallied up a sample of 158 NHL players.

At random, I chose five awards: the Frank J. Selke Award for Best Defensive Forward; Rookie of the Year; Playoffs MVP; the Lady Byng Trophy; and the Masterton Sportsmanship Award. I figured, if there's a way to calculate who "elite" hockey players are, award-winners is the most accurate criterion.

I counted all 158 players who had won one or more of these awards, being sure not to count people who had won two or more of them (or the same one twice), which is easy to do, since my browser highlights the links I've visited already.

Results: of the 158 players, 79 were born in the first six months of the year, 79 were born in the last six months. An exact tie (oddly enough, I also found exact ties in the 1982 and 1986 World Cups).

Here are the Wikipedia pages:

The monthly breakdowns reveal no pattern that I can discern:

Jan 12
Feb 12
Mar 18
Apr 13
May 12
Jun 12
Jul 19
Aug 10
Sept 8
Oct 15
Nov 10
Dec 17

I find it surprising that a high-level economist relies on "one graph that [he] found on the web" as the keystone of a New York Times article.

Perhaps badminton, Mr. Levitt? Shall I check?


David Kane

It appears that Dr. Levitt found his graph here.

Reliable data? Tough to know. Wonder why Dr. Levitt didn't follow the universal blogospheric practice of providing the link? You'll have to ask him. But I certainly wouldn't put an excessive amount of faith in a webpage that starts with this:

Do the world's best hockey players have a common cosmic signature? Does Mercury tend to be in a certain area of the sky when a professional hockey player is born? I am intrigued by that question and many, many others and what follows are the observations I have made during the course of the project. Stay with me, this gets very interesting.

During the 2001-2002 hockey season the National Hockey League was composed of 30 professional teams located in Canada and the United States of America, averaging 26 players per team (range 22 to 31), a total of 761 individuals. Birth information was gathered for each player including the day, month, year, city and country of birth. The time of each player's birth was not readily available and so the time for each player was set for noon on the day of birth, halfway through the planetary motions for the day. The birth information was input to Matrix Software's Win*Star Plus program which accurately produced a heliogram for each player.


A heliogram is a two dimensional arrangement of the planets around the Sun at the time of the birth of any individual. The position in the heliogram of Mercury, Venus, Earth and Mars for each player was transferred into Microsoft's Access database along with information about the heliogram "type" and the planetary patterns associated with the heliogram. The heliogram is a heliocentric view of our solar system and includes the positions of all the planets which orbit our sun including Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The heliogram is divided into 12 equal sized sectors each of 30° and labeled sectors 1, 2, 3 etc. in a clockwise direction from the East. Each planet in a heliogram has a planetary address; for example, a planetary address of 1215 means that the planet is in sector 12 at the 15th degree.

Good to know. Perhaps the next New York Times article will discuss the freakonomics of heliograms. I look forward to it.



Dear Dr. Levitt:

Thanks for looking into this further.

Congratulations to Bill L. Lloyd for his hard work and insights.

This kind of information could help answer questions about the relative importance of nature vs. nurture in different sports. For example, I would hypothesize that age cutoffs and birthdates would have little correlation with who makes the NBA (especially at center and forward), since height, which is highly heritable, is so overwhelmingly important and thus most players are drawn from the right edge of the height bell curve. If you are Manute Bol, the 7'-7" Dinka tribesman from the Sudan, you can enjoy a lengthy (if curious) career in the NBA, and lead the league in blocked shots, even though you didn't see a basketball in your life until you are 19.

On the other hand, perhaps a sport like soccer where players mostly are fairly average in size is more driven by nurture than nature than is basketball. It's hard to imagine the soccer equivalent of Manute Bol or of Nigerian Hall of Fame basketball center Hakeem Olajuwon, a seven foot who switched from being soccer goalie to a basketball player at about 16.

I don't pretend to know enough about soccer to make a prediction, but the relatively slow progress of the U.S. in the soccer World Cup, compared to say, Argentina in Olympic basketball, might suggest that culture is a bigger driver of success in soccer than in basketball.



On the other hand, the rapid decline of the USA Olympic basketball team since the awesome 1992 Dream Team may be an example of negative cultural influence on basketball success at the _team_ level.

The NBA stars who failed so ignominously at the 2004 Athens Olympics all grew up since the emergence of gangsta rap during the early years of the crack wars in the late 1980s. (NWA's "Straight Outta Compton" album was the first hit gangsta rap album in 1988.) Perhaps many contemporary NBA stars have internalized a lot of the chest-beating machismo celebrated in gangsta rap, which might help explain why they played as a team so much worse than the Dream Team of 1992, who were all full grown by the time gangsta rap arrived.

Obviously, this is speculative, but the timing is at least suggestive.

Stephen J. Dubner

In post No. 9 above, David Kane wrote:

No doubt I am an idiot, but after looking at the site for 15 minutes, I could not find a single piece of data that supported the factual claim that—- at the adult, elite level (i.e., World Cup soccer, NHL hockey)—- there is an month-of-birth effect.

The site I had referred him to, upon his request for further data about relative-age effect in the N.H.L., was this one:

I don't necessarily agree with David that he is an idiot, but the N.H.L. mention is right there under the site's first section, titled "Month of Birth and Elite Hockey," whose text reads, in part, as follows:

The Figure at left shows the distribution of birth-months of players in two Canadian major junior hockey leagues (the Ontario Junior Hockey League and the Western Hockey League). The data indicate that the probability of success in high calibre hockey is dramatically reduced for those born at the end of the year. Furthermore, among National League Hockey Players who were active in the early 1980s, about 40% were born in the first quarter of the year, 30% in the second, 20% in the third, and less than 10% were born in the final quarter.

In terms of playing at a high level, boys born in the last part of the year have a much lower chance than those born at the beginning of the year. The fact that January and December, which are juxtaposed, show such dissimilar results, suggests that it is not the weather during conception or birth that has made the difference.

Source: Barnsley RH, Thompson AH, Barnsley PE (1985). Hockey success and birth-date: The relative age effect. Journal of the Canadian Association for Health, Physical Education, and Recreation, Nov.-Dec., 23-28.

You could also look at another document posted on this site:

Go to Table 1, p. 150, or just search for "ice hockey."


Bill L. Lloyd

Mr. Levitt writes:

"If this rule had an important impact in determining who made the national youth soccer teams, then these early selection rules would play out to more long run success at the highest levels of soccer [at the pro level, this has not been proven at all; it's sheer conjecture on Levitt's part, both cause and effect]. The academic evidence is that these national teams are overwhelmingly made up of players born early in the calendar year, even on the age 21 and under teams, where a few months of physical development isn't likely to make a big difference."

For about the tenth time, *no one here is disputing whether national youth teams are more likely to feature players with early year births". This is universally agreed upon. What is being disputed is whether this effect lasts in adult pro leagues.*

Mr. Levitt has not provided a citation for his hockey graph that he "found on the web," so we cannot check it. I hope he didn't get it from the site Mr. Kane found it at -- some kind of astrology site -- but we'll wait for a citation from Mr. Levitt on that.

This isn't some hard-to-replicate laboratory study. It's counting birthdays of hockey and soccer players on Wikipedia.

We still have no good evidence that birth month has any effect on pro team makeup in hockey or soccer. If there is such evidence, I'd be glad to take a look at it.

Keep in mind that, on most topics, you can find a study that says A and a study that says the opposite of A. You can also find studies for anything in between.

That's why I like to use my own eyes when it's feasible, and remain skeptical of theories that don't sound right.

It just doesn't sound plausible to me that Mr. Levitt and Mr. Dubner's thesis is correct. January kids growing up might be borderline talented and slide onto all-star teams through adolescence boosted by their several-month advantage, but that effect would likely dissipate greatly in the post-adolescence college years, and then disappear by the time they're pros.

If the evidence shows otherwise, I'll look at Mr. Levitt and Dubner's explanations for the phenomenon.

But so far, the evidence doesn't show that there is any phenomenon.

It makes more sense to me that later-year kids would get lots of good practice growing up fighting against slightly bigger kids. This might offset (or more than offset) the self-esteem and extra coaching early-year borderline all-star kids would get.

Anyway, like I said on the previous thread: before going into complex reasons how the moon came to be made of green cheese, let's first determine whether it actually is made of green cheese.

Doesn't look like it from here, Mr. Levitt.


David Kane

Apologies for posting so many items in a row, but this is just too fun.

Although I can not vouch for the accuracy of this data, the site seems to provide a reasonable set of data on the 971 players in the NHL for 2005-2006. (I confess that it is not an astrology site, like the one used by Dr. Levitt, but please humor me.)

Now, if there are 971 players and 12 months in the year, how would we expect the birth-months to be distributed (ignoring days in the month, leap years and other details)? The null hypothesis would be 81 or so per month.

I had to play around with the options (use Sub-sort after checking Birth Month under personal info) and I could not figure out a way to provide a link to the results. But, by my quick count, here are the number of players born per month at the start and end of the year.

January 97
February 105
March 86

October 66
November 52
December 64

Pretty impressive for the Freakonomics claim! Here are the results by quarter of the year.

Jan-Mar 286
Apr-Jun 278
Jul-Sep 221
Oct-Dec 180

Note that the month values do not add up exactly to the quarterly sums. The interface is somewhat hard to use and I could easily be making a mistake.

Note also that I have ignored issues of country of birth (what should really be country-of-junior-hockey-playing). I also ignore the monthly birth rates for a matched population. (If more people are born in the first quarter of the year than the last quarter, we would expect to see the same being true for hockey players.)

In any event, I would conclude that there is evidence that current NHL hockey players are more likely to be born earlier in the year than later. I will leave significance tests for someone smarter than I to do.

Could this be due to cut-off dates for junior hockey? Perhaps. Key test might be to see if this pattern holds for different countries with other cut-off dates.


Bill L. Lloyd

Mr. Dubner writes:

"The Figure at left shows the distribution of birth-months of players in two Canadian major junior hockey leagues (the Ontario Junior Hockey League and the Western Hockey League). The data indicate that the probability of success in high calibre hockey is dramatically reduced for those born at the end of the year."

As we have already well-established on these threads, Mr. Dubner, *no one is questioning whether youth leagues of any sport have higher percentages of early-year births. Everyone on these threads agrees with that. We are only disputing whether this effect carries through into adulthood.*

Dubner, citing the study:

"Furthermore, among National League Hockey Players who were active in the early 1980s, about 40% were born in the first quarter of the year, 30% in the second, 20% in the third, and less than 10% were born in the final quarter."

I think that the "National League Hockey" cited in this study is not the NHL, but this Canadian junior governing body:

As the Wikipedia page shows, this league is for players aged 15-20 only.

Mr. Dubner, did you really think that 70% of NHL players were born in the first six months of the year? That stat alone would've set off my bullcrap meter bigtime.

Indeed, I just surveyed 158 NHL greats (see upthread) and came up with 79 born in the first six months of the year, 79 born in the last six months.

So we are back where we started with Levitt and Dubner. Their soccer data didn't hold up, and now their hockey data is, frankly, embarrassing.

Another sport you'd care to try, guys? Sorry to be snotty, but this is amateur hour economics and social science.


Stephen J. Dubner

Bill Lloyd,

Sorry, my mistake re my post 13 and your post 16. Thanks for correction. I'm pretty sure I have my leagues straight in the other cite, however:

(Table 1, p. 150.)

Bill L. Lloyd

David Kane,

Great work -- that site provides a 59%-41% split in 2005-06 NHL players. So this effect does exist in pro hockey.

I wonder if it's a new effect, given that my 158-player sample, which spanned many decades, found a 50-50 split.

Also, I apologize to Mr. Levitt and Mr. Dubner for my "amateur hour" comment. That was unnecessarily snotty.


Dear Mr. Kane:

Thanks. I wonder if this NHL data could turn out to be evidence for the kind of market inefficiencies that were identified by Bill James in the baseball business from 1975 onward, and have recently been exploited by Billy Beane and others in the big leagues -- e.g., for decades, good-looking athletes who swung at too many bad pitches were favored by scouts over less athletic hitters who got more walks because of their eagle eye for bad pitches.

I am informed by a reader that the NHL draft of Canadian players is largely restricted to 18 year olds and is quite biased toward those born early in the year due to their better performance in national and international junior showcases.

This drafting behavior could be an efficient response if born-early-in-the year players remain objectively better throughout their career. Or it could be a market inefficiency.

One of Bill James' first major finding a couple of decades ago was that in baseball draft picks out of college enjoyed better major league careers on average than draft picks out of high school.

Perhaps hockey has a market inefficiency because there isn't much of a Canadian college hockey culture, so you either make it at age 18 or you don't. And thus kids who are 18 and 364 days do better in the draft than kids who are 18 and 1 day on average.