"Parasite Prevalence And The Worldwide Distribution Of Cognitive Ability"
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July 06, 2010, 05:17 PM
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A reader writes in reference to the new paper in the Proceedings of the Royal Society, Parasite prevalence and the worldwide distribution of cognitive ability by Christopher Eppig, Corey L. Fincher and Randy Thornhill.

Steve, did you notice that The Economist finally decided to inform its readers about Lynn and Vanhanen`s IQ estimates?

When Dr Lynn and Dr Vanhanen originally published their IQ data, they used them to advance the theory that national differences in intelligence were the main reason for different levels of economic development. This study turns that reasoning on its head. It is lack of development, and the many health problems this brings, which explains the difference in levels of intelligence. No doubt, in a vicious circle, those differences help keep poor countries poor. But the new theory offers a way to break the circle.

Of course it`s only now that a politically correct explanation has been found 8 years later.

A subscription to The Economist is expensive, so subscribers should complain that the editors were covering up important information until a more polite spin could be found.

Actually, The Economist did cite "IQ and the Wealth of Nations" once before, in its May 15, 2004 issue (p. 26) when The Economist fell for that lowbrow Internet hoax about high IQ blue states and low IQ red states!

The Economist is doing a disservice to the long-term reputations of Eppig, Fincher, and Thornhill, since it`s an interesting paper, and I recommend that you read it. I don`t find anything novel about the idea that improvements in health are one of the reasons for rising raw IQ scores, but it`s a good point to reiterate.

Eppig et al don`t actually support the Economist`s spin, writing:

A nation of more intelligent individuals is likely to produce a higher GDP, but a wealthier nation is also more able to pay for public education, as well as public medical and sanitation services. An indirect link between education and intelligence may also exist, as a better-educated population may be more interested in public health measures—leading to increased IQ by reducing parasite stress—provided that education includes information about germ theory and hygiene. These sources of endogeneity must be considered when interpreting our findings (and see below). It should also be mentioned that we are not arguing that global variation in intelligence is only caused by parasite stress. Rather, variation in intelligence is probably caused by a variety of factors, including those we have mentioned here as well as factors that are yet unknown.

They make the point that I made last week:

We also propose a complementary hypothesis that may explain some of the effects of infectious disease on intelligence. As we mentioned, it is possible that a conditional developmental pathway exists that invests more energy into the immune system at the expense of brain development. In an environment where there has consistently been a high metabolic cost associated with parasitic infection, selection would not favour the maintenance of a phenotypically plastic trait. That is, the conditional strategy of allocating more energy into brain development during periods of health would be lost, evolutionarily, if periods of health were rare. Peoples living in areas of consistently high prevalence of infectious disease over evolutionary time thus may possess adaptations that favour high obligatory investment in immune function at the expense of other metabolically expensive traits such as intelligence.

Still, there are some problems with the paper.

Their model correlates national average IQ and "disability adjusted life-years lost" owing to infectious disease. (Unfortunately, I can`t find from following their footnotes where they got this data for 192 countries. I can only find it summarized by WHO by regions. If you can find the data for each country, please let me know.)

But, of course, the obvious question is: Does Singapore have a high average IQ because it has a remarkably low infectious disease burden for it`s tropical, sea-level, humid location, or does it have a remarkably low infectious disease burden because it`s run by high IQ and hard-working officials, such as Lew Kwan Yew?

In contrast, Afghanistan has terrible life expectancy and longevity figures compared to Singapore, indicative of terrible disease burden, despite having a climate rather like Colorado`s. (Afghanistan`s problem is less that its residents are empty-headed as that they are bloody-minded, with one of the most selfish, nasty cultures in the world.)

If Lagos has a similar climate to Singapore but worse disease burden, it that the cause or the symptom of the difference in average IQ? (Most likely both, of course, but it should be possible to allot causality.)

I think this key problem of causation could be tackled by using not the actual current disease burden but the predictions of a theoretical model of what disease burden would be based on the natural factors of each country. Eppig et al write:

The worldwide distribution of parasites is well known. Disease-causing organisms of humans are more prevalent in equatorial regions of the world and become less prevalent as latitude increases. Ecological factors contributing to this distribution include mean annual temperature, monthly temperature range and precipitation (e.g. Guernier et al. 2004). Similar trends of parasite distribution have been shown in other host species (e.g. M?¸ller 1998).

So, there must be models that would predict that Singapore and Lagos would have similar disease burdens. Therefore, use the theoretical predictions of disease burden to correlate with IQ, not the current actual disease burdens.

My commenter goes on:

Eppig et al report a strong correlation between infectious disease burden and national IQ, using either Lynn`s estimates or Wicherts revised estimates for sub-Saharan Africa.

Eppig et al repeat a number of related analyses to show that infection disease is the strongest, but they decided not to reanalyze the one variable that`s stronger than infectious disease: skin color (r = -.92). Their argument for why they can ignore the results of Templer & Arikawa (2006) is an amazing feat of intentional misunderstanding:

"Although Templer & Arikawa (2006) found a positive relationship between IQ and skin darkness, we will not use skin darkness in our analyses for three reasons: (i) although evidence suggests that skin darkness is a measure of historical infectious disease intensity over evolutionary time, it is unclear exactly what kind of infectious diseases it is indicative of (see discussion); (ii) Templer & Arikawa (2006) argued that the relationship between skin darkness and IQ is not causal; and (iii) Templer & Arikawa (2006) did not sufficiently explain why the association between intelligence and skin darkness exists. Without a reasonable theoretical framework for this association, we did not feel it was appropriate to compare it with other variables for which there is a better theoretical rationale."

Obviously the explanation for the relationship between IQ and skin color given by Templer & Arikawa (2006) was a population`s evolutionary history of cold climate with adaptation to UV exposure — a genetic model.

Again, the correlation found by Templer & Arikawa (2006) for IQ and skin color (-.92) exceeds that found for IQ and infectious disease (-.82), but this fact is avoided by Eppig et al and by The Economist.

Interestingly, Eppig el al note:

Mackintosh (2001) presented comprehensive evidence that skin darkness and the associated cellular components (e.g. melanocytes) have an important role in defending against infectious disease. Moreover, Manning et al. (2003) found that, in sub-Saharan Africa, rates of HIV infection were negatively associated with skin darkness. Manning et al. (2003) attributed this relationship in part to lower infection rates of other parasites, especially bacteria and fungi, that lead to tissue damage in the genital tract and hence increased opportunity for contracting HIV. Templer & Arikawa (2006) concluded that, despite the strong negative correlation between skin colour and average national IQ, there must be an unknown mediating factor accounting for both because there is no obvious reason for skin darkness to reduce IQ. Given the previous research linking skin colour to infectious disease (Mackintosh 2001; Manning et al. 2003), the unknown factor linking skin colour and IQ may be infectious disease.

Also, Eppig et al admit that the Usain Bolt-producing part of the world is a pretty healthy place but just isn`t all that super-smart.

The only world region in which this relationship [between IQ and disease] was not significant was South America [defined here to include the West Indies]. This exception may be owing to the presence of several outliers. The group of conspicuous outliers in which IQ was much lower than expected in the worldwide trend (figure 1) are all Caribbean countries (St Lucia, Dominica, St Kitts and Nevis, Antigua and Barbuda, Grenada, St Vincent and Grenadines, and Jamaica), which represent 4 of 23 nations in the South America analysis (St Lucia, Dominica, Grenada, and St Vincent and the Grenadines). Because these outliers are in the same geographical location, it is possible that local parasites that are not included in the DALY owing to infectious disease variable are causing these outliers.