Does national IQ depend on parasite infections? Er…

By Ed Yong | June 29, 2010 7:51 pm


[I was originally going to avoid this, but decided to do it for the critical analysis, because I suspect it will be widely but badly covered, and because I also suspect that very little of this coverage will point out the publication record of these authors. Which is worth pointing out. Have fun in the comments!]

Why do different countries have different IQs? You’d first answer probably has something to do with education, but a trio of US scientists have put forward a radically different hypothesis – international variation in intelligence is related to the prevalence of parasites in a country. As is, according to them, pretty much ever y major aspect of human culture (but more on this later)…

Christopher Eppig, Corey Fincher and Randy Thornhill (yes, that one) from the University of New Mexico have suggested that fighting off parasitic infections during childhood takes up valuable energy that might otherwise go towards the development of the brain. More parasites mean less well developed brains and weaker mental abilities.

To test their controversial idea, the trio collected average IQ values for countries all over the world using three separate sets of data. They also used the World Health Organisation’s data on global “disability-adjusted life years” (DALYs), a measure of a country’s disease burden that looks at the number of years of ‘healthy’ life lost by an average citizen because of poor health. They found a strong correlation between these two figures, both across all nations and within each continent (except South America).

They claimed that the prevalence of infectious diseases is the “most powerful predictor of average national IQ”, even after they adjusted the results for other factors, like each country’s temperature, GDP, literacy rate, enrolment in secondary school and more. They also suggest that this could help to explain the mysterious Flynn effect, where IQ increases sharply as a nation develops.

The very obvious caveat to all of this is that old adage that correlation is not causation. In this case, a link between infections and IQ tells us nothing about whether infected people grow up to be less intelligent, or whether intelligent people are less likely to become infected. Intelligence, after all, could affect one’s understanding of what a disease is, how to avoid it, and how to seek help for an infection.  And perhaps a third factor is at work here – higher education could lead to both greater intelligence and the knowledge to avoid common infections. Readers may enjoy trying to come up with alternative explanations of their own.

These problems become particularly astute when you’re looking for correlations between statistics that represent entire nations. This broad-brush ‘ecological’ approach tells us nothing at the individual level. In a given country, do children who acquire early infections grow up to have lower IQs? We simply don’t know.

In fairness to Eppig, Fincher and Thornhill, they say, “We are not arguing that global variation in intelligence is only caused by parasite stress.” They also frame their paper as a way of introducing a hypothesis and suggest ways of testing it. Fair enough, but they have supported their hypothesis with data that are, at best, inconclusive. As such, I wonder what this study is doing in a Royal Society journal rather than, say, Medical Hypotheses.

Indeed, as I alluded to earlier, this new paper is the latest in a long line of hypothesis-generating publications from Fincher and Thornhill linking parasites and infections to pretty much any sweeping aspect of human life you can think of. Through similar studies based on correlations at the national level, Thornhill and Fincher have suggested that infections are linked to individualism and collectivism, religious diversity, linguistic diversity, armed conflicts and civil war, and democracy and liberal values.  Like any attempt to explain very complex patterns of human behaviour through a single cause, this ought to raise an eyebrow. I’m raising two.

Reference: Proc Roy Soc B

CATEGORIZED UNDER: Medicine & health, Parasites

Comments (37)

  1. Roberta

    “These problems become particularly astute…”

    I assume you mean acute.

    Good article otherwise.

  2. Nathan Myers

    You’ve run out of eyebrows. What can you raise now?

  3. Raising other people’s eyebrows is fair game.

  4. Some Guy

    In before this article ends up reported completely misconstrued on the health segment on the local news television station …

  5. Eleanor

    I guess at some stage they’ll run out of traits within human societies to link to parasitism. I can’t read the paper yet, but if South America doesn’t fit the trend I’d have said that that was one theory dead in the water. Interestingly, that’s the third paper in this rich seam of correlations they’ve mined that has come out in the Royal Society journals.

    Could we have a sweepstake on what will relate to parasitism next? I’m putting a fiver on homosexuality.

  6. I am very skeptic about IQ in general, as I doubt it is a reliable method to measure something so complex and varied and difficult to define like intelligence (indeed I doubt such a method could possibly exist!). As for this article in particular: bad science. Bad, bad science

  7. National IQ is not necessarily indicative of ‘advanced’ civilization. I live in the DC area where the average IQ is probably one of the highest in the nation. However, you’d assume the direct opposite were you to try to drive around here with a rational approach to human behavior!

    Thanks for publishing this though Ed – as Some Guy said – when we hear it trumpeted (either approvingly or disapprovingly) on media outlets, we’ll know we can point folks back here to join the raised eyebrow parade.

  8. And here I thought the article would be about parasites in our brains being what makes us smarter! Finally, a good outcome of that classic sci-fi premise :-)

  9. @ Eleanor – Next on the list: music tastes, football ability, whether you put jam first or cream first on a scone, and whether you say “nuclear” or “nukular”.

    But seriously, homosexuality is probably a good call but given Thornhill’s resume, I’m going to go for rape.

  10. First, thanks for writing about this, Ed.

    Argh, not Randy Thornhill again! What a creepy sexist. My favorite quote from him is still the first sentence of Thornhill and Thornhill 1992 where they say “Psychological adaptation underlies all human behavior” to then justify their claims that rape is an adaptation. All? Really?

    After raising both my eyebrows, the only thing left to raise is my middle finger (at the study authors, not you :)). I wonder what psychological adaptation (or now parasite) underlies THAT behavior!

  11. They actually remind me of chiropractors, who claim that nearly all maladies are related to spinal misalignment.

  12. Analyzing national IQ averages may not be at the right level to investigate the possible link of parasites and intelligence. But what about numerous studies that show that de-worming kids raises their test scores?

  13. jdmimic

    Thank you Carl! I am not defending any prior work or statements of Thornhill or the others, but as scientists, we ideally should ignore the name on the paper and look at the science. In this case, the methods used are hopelessly blunt to make a valid determination. However, if we just look at the hypothesis, that parasitic loads can decrease intelligence, the hypothesis is reasonable and as Carl pointed out, they are not the first to make this claim. If this is valid, which there seems every indication that it very well could be, one would expect that countries in which the population as a whole shows high parasite loads would have lower IQ in general than those that have lower parasitic loads. Now, as Ed said, correlation does not prove causation, but it does provide a link that needs further explanation. If the link holds up, then all the numerous other factors need to be considered to distinguish proximal versus distal causal factors.

    But what is really bad science is to say that if the research comes from so-and-so it must be bad and toss it without a fair analysis of the actual work. Even if most of the work is indeed bad, there may still pieces that are valuable.

    I do agree that this work seems better suited to Medical hypotheses than a Royal Journal, but obviously the editors disagree.

  14. Where did anyone suggest that this research should be tossed simply because it came from so-and-so? Point it out to me if I missed it.

    All anyone’s saying is that the authors’ tendency to pin anything and everything on parasites is a bit suspect, and that this idea is not well-supported enough to qualify as more than a hypothesis.

    Here’s another blind stab at accounting for the link: Societies with higher IQs have higher standards of just about everything: lifestyle, education, medical care, sanitation, cleanliness, construction, transportation, etc. I’d imagine that just about all of these factors have at least some bearing on the prevalence of parasite infections.

    The point is, there is an extremely complex set of causal relationships that includes both IQ and parasite infections, and the burden of proof is on the authors to show that this is more than just a red herring.

  15. jdmimic

    The bearing of the studies on commentaries of the article being reported on is that the hypothesis of Eppig et al. is actually supported by other research.

  16. jdmimic

    @ Brian: You’re right, no one openly said the research should be tossed simply because it came from so-and-so. The very fact that I said “and so-and-so” should have clearly indicated I was alluding to a general sort of behavior.

    But to give examples of what I was indicating: Ed’s commentary has several of these allusions. the “(yes, that one)”, “As is, according to them, pretty much ever y major aspect of human culture “, and his whole last paragraph are designed to cast doubt more on the authors than they are on the research. In the comments, there are such comments as “creepy sexist”, “they remind me of chiropractors,” “Could we have a sweepstake on what will relate to parasitism next?” and of course Ed’s comment at no. 9. These comments are all directed at the authors and really have very little to do with the hypothesis in question.

    Admittedly, when a set of authors goes overboard and try to pin all of human civilization on one thing, they open themselves up to derogatory comments and quite fairly so. However, it seems clear to me that when one goes down this road, serious examination of further research becomes much more difficult. From all the research on flawed perception that Ed has reported here (for which I am very grateful), I am sure he understands this.

    All the factors you mentioned as possible causes do indeed play a role. The real question though is not what single factor is most important (and indeed Ed pointed out that Eppig et all. don’t claim that parasitism is the only factor), but how they interact. What is the mechanism by which medical care, education, sanitation, etc. cause changes in intelligence? Could it be that all of these factors are important for reducing parasite loads, which then affects intelligence? I am not saying this is necessarily the case, but this is what I mean about distinguishing proximal versus distal causes. Sanitation and medical care for instance are not in and of themselves direct causes of intelligence alterations. They are affecting something else which really is making the changes.

    But to say that because the authors have linked everything under the sun to parasites does not really affect the validity or invalidity of any particular hypothesis. i would also point out that many of the things they have said are in fact intertwined. They do influence each other, so it is not implausible that something that affects one has an influence on the others. Is it as big as they say? I doubt it. But it deserves critical (as in clear and logical) examination.

  17. I’m also pleased with the links that Carl chipped in with. To clarify, I’m not arguing against the basic hypothesis that parasites can influence mental development. I’ve written about parasites extensively and I’m well aware of the significant effects they can have. My point is that the paper puts this hypothesis forward alongside data that aren’t really strong enough to support/refute it – “hopelessly blunt” as Jdmimic says.

  18. Adriana

    I, too, am weary of any correlations with IQ because it is not even clear what IQ tests really measure, and measuring such a complex trait as intelligence is likely to run into imprecisions due to many different factors. Regarding this comment: “Intelligence, after all, could affect one’s understanding of what a disease is, how to avoid it, and how to seek help for an infection. And perhaps a third factor is at work here – higher education could lead to both greater intelligence and the knowledge to avoid common infections.”, this comment is not quite fair because the authors propose that infections in EARLY childhood would have an effect of brain development, and babies and little children, even those with the highest IQ, will not have any clue how to avoid getting infected by parasites. [That’s a very fair point – Ed]

    Even if parasitic infections do not directly cause a lower IQ, if the correlation holds water, it is an interesting observation that maybe indicative of general sanitary conditions or other environmental factors that may have a more direct effect in IQ.

    Perhaps your criticism would be more productive if you stick to criticizing the actual data: if the data does not pass muster, and there is no strong correlation, then there is no need to criticize the scientists themselves, for having linked parasites to everything; even if they have done that systematically, there may be some truth in some of their findings.

    I’m in complete agreement of the absolute best data that needs to be collected before any conclusions can be reached: IQ of children, teenagers or young adults who acquired parasitic infections early in childhood, within one country and within a given socio-economic tier.

  19. Nathan Myers

    I’m also suspicious of IQ tests, but they do measure something repeatably. It’s hard to argue that something so well correlated with other objectively desirable qualities is itself not desirable, whatever the hell it is. Ed hints obliquely at psychological effects of parasitic toxoplasmosis infection, which as it affects judgement could easily affect IQ. If it didn’t, that would only indict IQ itself; you would then need to look for the other correlates, but IQ would probably turn out to be a reasonable proxy. There’s no reason to assume that if toxoplasmosis can affect behavior through neural meddling, other parasites would be less capable.

    We might say it’s unfortunate that the authors who published the paper are the ones who did, as no reputable researcher wants to be seen corroborating their work.

  20. deggbfsrf

    This article made me smile
    i really doubt that parasites affect american people and yet… :)
    There is no cure or disease of simple stupidity
    They have to pay me, if you want my own versionand explanation.

  21. Jeff Bates

    The article is at . Please read the article abstract before commenting. ( link does not work now.)

  22. Vaccinations make us smarter!?

  23. Matt Greenall

    You were right in anticipating the way this story would be covered. Look at the Guardian, and at the CV of the Emeritus Professor from Ulster they got to comment on the story:

    I want to ask people who know about IQ (I don’t): given that we don’t know exactly what we measure, do we know whether whatever it measures is measured in a stable and consistent way across countries. In other words are comparisons of average IQ between countries definitely fair comparisons? Thanks.

  24. M Holtz

    ***Why do different countries have different IQs? You’d first answer probably has something to do with education***

    And natural selection in different environments and cultures? Have you read ‘The 10,000 Year Explosion: How Civilization Accelerated Human Evolution’. It seems the Chinese will answer this soon:

    “It is about to embark on a search for the genetic underpinning of intelligence. Two thousand Chinese schoolchildren will have 2,000 of their protein-coding genes sampled, and the results correlated with their test scores at school. Though it will cover less than a tenth of the total number of protein-coding genes, it will be the largest-scale examination to date of the idea that differences between individuals’ intelligence scores are partly due to differences in their DNA.

    Dr Yang is also candid about the possibility of the 1,000-genome project revealing systematic geographical differences in human genetics—or, to put it politically incorrectly, racial differences. The differences that have come to light so far are not in sensitive areas such as intelligence. But if his study of schoolchildren does find genes that help control intelligence, a comparison with the results of the 1,000-genome project will be only a mouse-click away.”

  25. M Holtz

    ***I want to ask people who know about IQ (I don’t): given that we don’t know exactly what we measure, do we know whether whatever it measures is measured in a stable and consistent way across countries. In other words are comparisons of average IQ between countries definitely fair comparisons? ***

    Matt Greenall,

    Garrett Jones & Joel Schneider discuss the research here:

    “Considerable effort has gone into producing nonverbal IQ tests that can be used in any culture. These “culture-fair” or “culture-reduced” IQ tests have been shown to predict important life-outcomes with validity coefficients comparable to traditional IQ tests designed for specific populations…

    As we note below, the correlation between national average IQ and GDP per worker is essentially unchanged if we only use data from such “culture-reduced” tests. Unlike traditional IQ tests that measure a very diverse set of cognitive abilities, culture-reduced IQ tests necessarily measure a much smaller number of abilities, focusing on nonverbal reasoning and novel problem-solving. Fortunately, the types of tests that lend themselves to cross-cultural research correlate very highly with the overall scores from traditional IQ tests…

    For our purposes, it does not matter if one believes that IQ tests are valid measures of whatever “real intelligence” is (if there is such a thing as “intelligence”). The tests measure a set of skills that appear to be very advantageous in societies with modern economies. Unlike other measures of human capital such as reading comprehension and mathematical reasoning tests, culture-fair IQ tests have no literacy prerequisites. Because the tests are nonverbal, the test items are the same for everyone and thus results are more comparable across language groups and cultures…

    Our estimate, which we round to unity, provides a number of insights. First, it shows that LV’s national average IQ measures are useful for predicting more than just cross-country productivity differences, cross-country growth rates (both positive correlations), cross-country suicide rates (also a positive correlation (sic): Voracek 2004, 2005), and other cross-country factors. We have now shown that they are also useful for predicting the age-and-education adjusted wages of the average immigrant coming from her home country to the United States.5 This is surely evidence that national average IQ is an important predictor of what Hanushek and Kimko (2000) call “labor quality.”

    Further, we have shown that the estimate is quite close to conventional
    microeconometric estimates of the IQ-wage relationship.6 Whatever an IQ test can tell us about worker wages, it appears to be measuring the same thing across countries as within countries. This is confirmatory evidence that cross-country IQ comparisons are indeed possible..”

    ‘IQ in the Production Function’

  26. Matt Greenall

    Thanks M Holtz, that answers my question.
    I’m still a bit worried about this: “The tests measure a set of skills that appear to be very advantageous in societies with modern economies” given that we are talking about countries, and particularly vulnerable communities within those countries, that have less “modern” economies.
    None of which detracts from the point made by Ed Yong to start with that this is no more than an ecological correlation, and is pretty flaky whatever the reliability of the measures.

  27. Justin

    There is a further unreported (possible) correlation that could drive the observed correlation: that quality of national education may correlate with disease penetrance in a given country (due to government spending & social policies). This would also drive a correlation between IQ and disease prevalence as reported. I think the original article is terribly under controlled and short sighted. Far more likely causal factors (by parsimony and common sense) are available but left under the extremely lumpy carpet.

  28. bottle

    > And perhaps a third factor is at work here – higher education could lead to both greater intelligence and the knowledge to avoid common infections

    That’s a contentious area of course. I would say there’s strong evidence against education having any significant effect whatsoever on adult IQ. Certainly some would disagree. I consider their arguments very weak. They presumably don;t.

    > Intelligence, after all, could affect one’s understanding of what a disease is, how to avoid it, and how to seek help for an infection.

    Yes – but I think it’s about social behavior not so much individual behavior. I mean social behaviors such as having a clean meat supply… excellent delivery of immunizations and the economic productivity to afford that with ease… having the kind of technical-organizational ability and economic surpluses that permit something like the programmatic eradication of malaria from the southern USA. And so forth.

  29. bottle

    However, maybe HIV is one example of infection where individual knowledge about how to avoid the infection (both in terms of sex and needle sharing) is crucially important. And not only is that knowledge important, but the variance in that knowledge is highly salient.

  30. Matt Greenall

    The claim on HIV struck a bum note with me. I’ve worked on designing and evaluating HIV prevention programmes in quite a few poor (and highish HIV prevalence) countries, and with the most vulnerable groups. HIV transmission can be high even where knowledge is good. The ability to act on the knowledge is constrained by a mix of many things including norms around sexual behaviour and condom use, lack of belief in the information, coercion, gender inequality (and lack of power to negotiate “safe” sex), lack of access to comprehensive sex education and to good health services including condom availability & needle exchange, and perhaps also the fact that the threats of HIV and AIDS are not felt to be as “immediate” as other problems people in poor countries face. There’s also a problem of lack of tailored information and services for groups with higher risk behaviours, who are often stigmatised.

    Unfortunately this doesn’t stop many, many HIV prevention programmes continuing to focus on disseminating information.

    Needless to say I was particularly wound up by the quote from Lynn at the end of the Guardian article!

  31. Mark Doherty

    The evidence that IQ is modifiable by education and social background is substantial. The evidence that parasitic infection correlates with socio-economic status is also substantial. We’ve just finished an immunological survey of schools in Addis Ababa, Ethiopia.

    That shows that children from free, government schools are significantly more likely to have parasitic infection than children from private schools – even where both schools part of the city. No prizes for guessing which children perform best on standardized tests. Yet attendance at such a school is no indicator of raw intelligence – it is determined by the parents’ choice and willingness/ability to pay, long before the child even enters the school.

    As Ed notes correlation is not causation. But what parasitic infection correlates with in this case, is probably not IQ per se, but socio-economic status, which affects a host of other factors including performance in IQ tests.

    In other words the claimed correlation may well be true, but still be meaningless.

    regards, Mark

  32. NB

    maby people who have a higher IQ are less likely to get a parasite rather then the lack of parasites making their IQ higher?

  33. AW

    Did anyone actually read the article?

  34. Good article and good comments … particularly Zimmer’s links. Even if this
    correlation turns out to be causal, and Zimmer’s links strenthen this
    plausibility, it has nothing
    to do with the Flynn effect … which
    continues in industrial countries long after good plumbing has dropped
    parasite and other water born diseases to a minimum. In any event, Flynn
    has done a pretty good job in his book “What is Intelligence” of explaining
    the effect and its myriad subtleties.

  35. The Zimmer links look convincing until you look at:

    Which does a thorough review and finds the evidence that
    helminth treatment improves either growth or cognitive skills is
    at best, dismal.

  36. You nailed it. Correlation is not causation. It’s all too easy to think of ways that parasitical infection could be a proxy for other factors — malnutrition, lack of mentally stimulating environment, poor education, and even poverty of conceptual language. Then there are the problems of cross-cultural IQ testing…



  37. Before the trolls even start, (you know who you are), I purchased the GoFlex Satellite for my the wife and I (two 32GB Wifi iPads) and using it on our trip to Las Vegas was great but the one gripe I have is having to use the GoFlex Media app. Anyone know of a app that works that I can use while at home (while my wife watches Housewives of [fill in the blank]


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