Lower IQs found in disease-rife countries, scientists claim
Energy can be diverted away from brain development to fight infection, explaining 'lower intelligence in warmer countries'
People who live in countries where disease is rife may have lower IQs because they have to divert energy away from brain development to fight infections, scientists in the US claim.
The controversial idea might help explain why national IQ scores differ around the world, and are lower in some warmer countries where debilitating parasites such as malaria are widespread, they say.
Researchers behind the theory claim the impact of disease on IQ scores has been under-appreciated, and believe it ranks alongside education and wealth as a major factor that influences cognitive ability.
Attempts to measure intelligence around the world are fraught with difficulty and many researchers doubt that IQ tests are a suitable tool for the job. The average intelligence of a nation is likely to be governed by a complex web of interwoven factors.
The latest theory, put forward by Randy Thornhill and others at the University of New Mexico, adds disease to a long list of environmental and other issues that may all play a role in determining intelligence. Thornhill made the news in 2000, when he coauthored a provocative book called A Natural History of Rape in which he argues that sexual coercion emerged as an evolutionary adaptation.
Writing in the journal, Proceedings of the Royal Society, Thornhill and his colleagues explain that children under five devote much of their energy to brain development. When the body has to fight infections, it may have to sacrifice brain development, they say.
In the American South, for example, hookworm, an energy-sapping infection that can cause cognitive impairment, was a giant problem until John D. Rockefeller funded a campaign against it starting in 1909. Poor Southerners seemed to have a lot more pep, physical and mental, once they started wearing shoes and taking other steps to avoid hookworm.
Hookworm is still a big problem in some of the warm-weather parts of the world. I'm sure there are other nasty parasites, and they tend to be more common in the tropics.
As Greg Cochran and Paul Ewald pointed out in the 1990s, there are probably numerous chronic infections that don't attract as much attention as major acute ones, but do often add up to trouble. A lot of things in the modern world, such as clean tap water, probably diminish their impact. Little kids get a lot of antibiotics these days for acute infections like earaches. The antibiotics might be killing off low-level infections at the same time. Who knows?
Likewise, Darwinian selection under conditions of heavy infectious disease burden will tend to be oriented toward improving the immune system more than raising intelligence, which will tend to have long term effects on tropical populations.
To test the idea, Thornhill's group used three published surveys of global IQ scores and compared them with data from the World Health Organisation (WHO) on how badly infectious diseases affect different countries. The list included common infections, such as malaria, tetanus and tuberculosis.
The scientists found that the level of infectious disease in a country was closely linked to the average national IQ. The heavier the burden of disease, the lower the nation's IQ scores. Thornhill believes that nations who have lived with diseases for long periods may have adapted, by developing better immune systems at the expense of brain function.
"The effect of infectious disease on IQ is bigger than any other single factor we looked at," said Chris Eppig, lead author on the paper. "Disease is a major sap on the body's energy, and the brain takes a lot of energy to build. If you don't have enough, you can't do it properly."
"The consequence of this, if we're right, is that the IQ of a nation will be largely unaffected until you can lift the burden of disease," Eppig added.
"It's an interesting and provocative finding," said Geraint Rees, director of the UCL Institute of Cognitive Neuroscience. "It explains about 50 to 60% of the variability in IQ scores and appears to be independent of some other factors such as overall GDP."
"The authors suggest that more infectious disease could lead to lower IQ scores through an impact on brain development. This is an interesting speculation, but the data don't prove it one way or the other," he said. "A bigger problem is that it might be driven by a third factor, that affects both infectious disease prevalence and IQ test scores."
Right. Multicollinearity is always a problem with correlation studies, and this topic especially: are lower IQs in the tropics caused by tropical diseases or by the tropics that cause the tropical diseases themselves?
For example, I've long hypothesized that one general problem that brains need to deal with is shedding heat and that is of course a bigger problem in the tropical than the temperate world.
How to dissipate heat generated by computer chips is a huge issue in computer design. As I type on my laptop computer, the surface of the machine that is touching my wrists is about 100 degrees F. The bottom of my laptop must be 150 degrees or more. A fan is running full speed to shed heat to keep the CPU chip from melting. My office is heating up from the combination of my PC and myself, both working hard. I have just now opened my window to disperse the heat. It is a cool evening here, so the temperature is palpably dropping by the minute. If I was in a tropical climate, I'd need to turn on the air conditioning or start the fan or whatever.
Intel had driven up the power of CPU chips largely by increasing the clock speed, but when they hit four gigahertz, Intel found that chips were melting down. So, Intel had to revamp massively and find other way to follow Moore's Law, such as multiple cores.
Similarly, your brain generates more heat when you are thinking hard than when it is idling.
Not surprisingly, skull shapes seem to be somewhat related to heat dissipation problems. Eskimos have round heads to conserve heat, while Kenyan marathoners tend to have narrow heads with a lot of surface area that dissipate heat more easily.
For reasons that are unclear, IQ scores are generally rising around the world. Thornhill suggests monitoring rates of infectious diseases in nations as they develop, to see if they decline and IQ tests scores rise.
Singapore would seem to be an example of a tropical place where public hygiene, antibiotics, air conditioning, education, and so forth combine in a virtuous circle.
Richard Lynn, professor of psychology at Ulster University, and author of the 2002 book, IQ and the Wealth of Nations, said disease and IQ is a two-way relationship, with low national IQs being partly responsible for widespread infectious diseases.
Right, it's hard to get started turning, say, Equatorial Guinea into Singapore without Singaporeans to get you started.
Here's the abstract of Thornhill's paper:
In this study, we hypothesize that the worldwide distribution of cognitive ability is determined in part by variation in the intensity of infectious diseases. From an energetics standpoint, a developing human will have difficulty building a brain and fighting off infectious diseases at the same time, as both are very metabolically costly tasks. Using three measures of average national intelligence quotient (IQ), we found that the zero-order correlation between average IQ and parasite stress ranges from r = −0.76 to r = −0.82 (p < 0.0001). These correlations are robust worldwide, as well as within five of six world regions. Infectious disease remains the most powerful predictor of average national IQ when temperature, distance from Africa, gross domestic product per capita and several measures of education are controlled for. These findings suggest that the Flynn effect may be caused in part by the decrease in the intensity of infectious diseases as nations develop.
Afghanistan would be the leading example of a cold winter place that seems pretty dim, perhaps due to disease burden. It's second in the world in infant mortality. (The rest of the Worst 20 are black African countries, while Afghans are, as Daniel Dravot notes in the Man Who Would Be King, a bunch of more or less white people. But how did that Civilizing Mission thing work out for you, Danny boy?)
James Michener's 1963 novel about Afghanistan, Caravans, has an Afghan leader arguing, with some pride, that while children die like flies in Afghanistan, if they survive past childhood, they grow up to be tough, mean bastards with well-tuned immune systems.
But, it's stupid of Afghans to have so much disease burden. So, I'm not sure that says much about causality. Are they so knuckleheaded because they are sick so much, or are they sick so much because they are so knuckleheaded?
But, even when it proves hard to determine ultimate causation from correlation studies, correlation itself is worth knowing. The general rule is that, as Kingsley Amis said in Lucky Jim: "There was no end to the ways in which nice things are nicer than nasty ones."