Climate and Disease

In a recent study, which controlled for spatial autocorrelation, the temperature and parasite burden hypotheses for differences in national IQ were found to be the most consistent ones. The hypotheses tested were: Distance from the environment of evolutionary adaptedness, Temperature, Nutrition, Parasite Burden, Education, and GDP.

Hassall and Sherratt, 2011. Statistical inference and spatial patterns in correlates of IQ.

Correcting for SAC in conjunction with exhaustive model selection enables us to circumvent the twin problems of spatial autocorrelation and collinearity among variables. This permits the most comprehensive and statistically rigorous assessment of six potential hypotheses explaining variation in geographical patterns in IQ that has yet been conducted. When a comprehensive model comparison was conducted to analyse national variation in IQ scores, then infectious and parasitic diseases (IPD) and temperature (mean temperature of the coldest quarter) were the only two variables consistently included in models. Mortality and morbidity resulting from nutritional deficiencies (Nut), GDP, and distance from the environment of evolutionary adaptedness (DEEA) also feature in some of the best fitting models. However, it is worth noting that DEEA becomes far less important in models after controlling for SAC. This is not surprising given that the variable itself is, by definition, autocorrelated across space. It seems likely that the distance from the environment of evolutionary adaptedness has no causal link with national mean IQ.

The case for an effect of infectious and parasitic disease burdens influencing national IQ has been made elsewhere (Eppig et al. 2010). Previously, the relationship between temperature and national mean IQ has been explained in terms of the greater cognitive demands of surviving in colder environments (Templer & Arikawa 2006). Given the strength of evidence for the physiological effects of disease, it may be that temperature is acting not through an impact on the environment but through an impact on the interaction between humans and their diseases. Temperature influences a number of disease-related parameters such as disease distribution (Guernier, Hochberg, & Guégan 2004), transmission seasons (e.g. malaria, Hay, Guerra, Tatem, Noor, & Snow 2004), the ability of insect vectors to transmit diseases (Cornel, Jupp, & Blackburn 1993) and the development and survival of parasites and host susceptibility (Harvell et al. 2002). It may be that temperature is having an effect on national mean IQ by mediating the response to infectious diseases rather than via environmental complexity.

The authors’ tentative interpretation was that the physiological effects of parasite burden are the proximate cause of differences. Of course, as Eppig et al. 2010 noted, such effects could translate into genetic differences if the rates of infection were consistent over evolutionary time. This brings to mind a passage for Race and Reality:

Driven from their conflicting defenses of isolation and lost ruins, some equalitarians finally retreated to the excuse of climate and disease, to the argument that tropical maladies and the heat were enough to account for the Negro’s condition. I knew of no scientists who advanced this argument, but it was frequently heard from laymen.

Here again one needed only to reply that, on the one hand, there were many parts of Africa where the climate was good and, on the other hand, other parts of the world which had produced great civilizations where the climate was bad. Moreover, for a hundred years the Negro had been free of both tropical diseases and the incubus of climate in the old ex-slave settlement at Chatham, Ontario. Yet his performance there on intelligence tests followed the standard pattern.{45} In fact tropical diseases no longer could be blamed for the Negro’s relative performance in the Southern United States.

The truth of the matter was that whatever influence climate and disease may indeed have had upon the Negro over tens of thousands of years, the result had by now become innate through evolutionary processes. I could paraphrase Nathaniel Weyl and state that “the fundamental barrier is less the action of climate and disease on the living generation than its cumulative action, over an immense time span, in forming the race.”

A strictly environmental explanation is complicated by the correlation between cranial capacity and temperature/parasite burden. Though, presumably, differences in cranial capacity could have a proximate environmental origin, that data is more consistent with the hypothesis that differences are largely genetic in origin (Sparks and Jantz, 2002).


(Bailey and Geary, 2010. Hominid Brain Evolution: Testing Climatic, Ecological, and Social Competition Models).

Anyways, the study offers more support for the biologicality and deep history of (some of the) regional GMA differences. I find that conclusion inescapable.

References

Sparks and Jantz, 2002. A reassessment of human cranial plasticity: Boas revisited

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4 thoughts on “Climate and Disease

  1. I recall the Eppig paper showed Carribean & South American populations had similar parasite burdens, but the Carribeans nonetheless scored well below the SA populations.

  2. Interesting they rebut something that Scott Barry Kaufman was pushing last year.

    “There has been debate in the literature over the competence of IQ tests to accurately measure intelligence over a range of education or literacy
    levels (Barber 2005), with some researchers claiming that
    global variation in IQ is entirely an artefact of varying literacy
    (Marks 2010). We find no evidence to support this.”

    Bob Wiliams also provided a response at the time.

    http://www.iqscorner.com/2010/09/williams-comments-on-scott-barry.html

  3. Parasite burden on mental and physical energy was widely observed in the American South a century ago. The Rockefellers funded a huge campaign to wipe out hookworm in the South, which appears to have done Southerners a lot of good.

    • I remember first hearing about that in a parasitology class (taught in the northeast); the professor quoted Mark Twain on the subject. Did you read Eppig, Fincher, and Thornhill (2011)?

      In this study, we tested the parasite-stress hypothesis for the distribution of intelligence among the USA states: the hypothesis proposes that intelligence emerges from a developmental trade-off between maximizing brain vs. immune function. From this we predicted that among the USA states where infectious disease stress was high, average intelligence would be low and where infectious disease stress was low, average intelligence would be high. As predicted, we found that the correlation between average state IQ and infectious disease stress was − 0.67 (p < 0.0001) 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. ( "Parasite prevalence and the distribution of intelligence among the states of the USA“)

      It helps make sense of Ryan, Bartels, and Townsend’s findings. (Ryan, Bartels, and Townsend, 2010. Associations between climate and IQ in the United States of America.)

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