There is a theory that development of immune response to sustained, chronic infection comes at the expense of IQ. Sounds implausible? It did to me as well. But hold your judgment until we explore some fascinating facts.
According to this theory, chronic parasitic infections (and this include not only worm and amebic infections, but all infections, including bacteria and viruses) causes the development of an elevated immune response. But this is not cost-free (nothing is!). The antibodies and cells that constitute the immune response are made of protein, and to synthesize and maintain them over long periods of time is energetically expensive. Now consider a baby (let’s say up to 5 years old), who is growing rapidly. Among the most critical energy-hungry organs undergoing development at this age is the brain.
The brain is the most complex and costly organ in the human body. In human newborns, the brain demands 87 per cent of the body’s metabolic budget, 44 per cent at age five, 34 per cent at age ten, and 23 per cent and 27 per cent for adult males and females, respectively (is that why women are somewhat smarter?). Presumably, if an individual cannot meet these energetic demands while the brain is growing and developing, the brain’s growth and developmental stability will suffer. So it stands to reason that a child who is afflicted with a chronic parasitic infection would have a over-developed immune response, and an under-developed brain. Diarrheal diseases may impose the most serious cost on their hosts’ energy budget. First, diarrheal diseases are the most common category of disease on every continent, and are one of the two top killers of children under five, accounting for 16 to 17 per cent of all of these deaths worldwide. Second, diarrhea can prevent the body from accessing any nutrients at all. If exposed to diarrheal diseases during their first five years, individuals may experience lifelong detrimental effects to their brain development, and thus intelligence.
This is intuitively plausible, but the data is not straightforward. IQ is associated with a fully developed brain, with genetics, with nutrition, with education. But with infection? Even if you could show a relationship between IQ and parasitic infection burden, how do you separate this effect from nutrition or education? One could argue that people with poor nutrition or inferior education are also the ones at risk of poor hygienic standards and chronic infection with parasites of all kinds. Indeed, several investigators made that correlation, with education and nutrition. But remember: correlation does not mean causation.
The Evidence for Infectious Burden as a Major Factor in IQ.
Christopher Eppig and his colleagues, of the University of New Mexico in Albuqerque, published a fascinating study in the Proceedings of the Royal Society/biological Sciences (June, 2010) examining this issue. In brief, they have shown a very significant negative correlation between the parasitic burden of a country (using WHO data) and a national average IQ (using a validated database) –the higher the parasite burden, the lower the IQ. To deal with the conundrum of confounding factors such as nutrition and education they showed, using statistical methods, that infection is an independent factor, even after accounting for education and nutrition. Furthermore, when they analyzed level of education in the same way they found that after accounting for infection burden, national education level was not statistically correlated with national IQ. In fact, the correlation between infectious disease and average national IQ was higher than that of any other variable for which there is a previously proposed causal explanation.
What about closer to home, here in the U.S?
Eppig just published a follow- up paper in the March issue of the journal Intelligence showing that the negative correlation between average state IQ and infectious disease stress was highly significant across the 50 states. Furthermore, when controlling the effects of wealth and educational variation among states, infectious disease stress was the best predictor of average state IQ. And which states have the highest infection burden? The ones with the most under-funded public health systems –the South.
What can we learn from these studies?
1. In the developing world we should be digging latrines and distributing mosquito nets, rather than finance white elephant projects that only enrich the kleptocratic elite, with zero impact on the lives of the people.
2. At home, we are cutting the public health funds at our own peril. We are creating a permanent underclass with no prospect of upward mobility. Some people may like the idea; people with no education and low IQ don’t ask for much, don’t know what their rights are, are easily brainwashed, and don’t vote. But is it in the national interest?
These studies by Eppig are excellent, but the ultimate evidence for a causal, rather than a correlative, relationship between infectious burden and IQ would come from longitudinal studies. Follow newborns to age 20 or so, and correlate their infection rate, especially at young age, with their intelligence quotient. Unfortunately, given the political climate we are in, what are the odds that such a study will be funded?