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.

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  1. SteveSailer says:

    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.

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  2. SteveSailer says:

    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.

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  3. 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:

    http://www.socialproblemindex.ualberta.ca/relage.htm

    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:

    http://www.freakonomics.com/pdf/RelativeAgeEffectSportsMusch2001.pdf

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

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  4. 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:

    http://www.socialproblemindex.ualberta.ca/relage.htm

    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:

    http://www.freakonomics.com/pdf/RelativeAgeEffectSportsMusch2001.pdf

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

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  5. Bill L. Lloyd says:

    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.

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  6. Bill L. Lloyd says:

    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.

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  7. David Kane says:

    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 Sportsnet.ca 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.

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  8. David Kane says:

    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 Sportsnet.ca 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.

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