Does infectious disease have an adverse impact on human intelligence? A new study posits that it does.

Prior posts have noted the importance of providing clean water and sanitation services to the human population in order to lessen the impact of disease on human health and well-being.  Now researchers are exploring how disease may impact human intelligence.

Human intelligence is a fuzzy concept.  Of what it consists, and how it is measured, are highly controversial topics.  For purposes of the thesis described below, the researchers assumed that IQ studies were a good indicator of intelligence.  Whether that is a valid assumption is not assessed in this blog post.

If one uses IQ tests as a measure of intelligence, then one will note that intelligence seems to be higher in some locales than in others, and also that it has been rising in recent decades.  Researchers noted that the brains of newly born children require 87% of a child's metabolic energy; at five years of age, 44%.  In adults, although the brain accounts for 2% of the body's weight, it consumers 1/4 of its energy.  So, any competition for this energy could possibly damage brain development, especially among the young.  Parasites and pathogens compete for that energy.  Some feed on the host's tissue, or hijack its molecular machinery to reproduce.  Those that live in the human intestine, interfere with the absorption of food.  All parasites and pathogens provoke (to varying degrees) the host's immune system, which diverts energy from other tasks.

The researchers estimated the disease burden using WHO data on disability-adjusted life years lost caused by 28 infectious diseases.  Such data exists for 192 countries.  The IQ scores came from work done a decade earlier by British and Finnish researchers who analyzed IQ studies from 113 countries; they also used subsequent work by Dutch researchers. 

The researchers found an inverse correlation between a country's disease burden and its average IQ.  At the bottom on the average IQ scale were Equatorial Guinea, St. Lucia, Cameroon, Mozambique, and Gabon.  These countries also have the highest burden of infectious disease.  At the top on the IQ scale were Singapore, South Korea, China, and Japan.  These countries have a relatively low level of disease; the U.S. and many EU countries follow the top group. 

The correlation is about 67%, and the chance about the result being random is 1:10,000 [take all statistics with a grain of salt since many research reports use faulty or lousy statistical methods]. 

Since correlation is not causation, the researchers went about eliminating other possible explanations.  Other studies have posited income, education, low levels of agricultural labor (which is allegedly replaced by more mentally stimulating jobs), climate (the challenge of surviving cold weather might select for those with greater intelligence), and distance from humanity's African roots [see prior posts] (novel environments were speculated to encourage greater intelligence) as explanations for national differences in IQ.  The researchers noted that all of these, but the last, were also likely to be linked to disease.  Using statistical analysis, the researchers either were able to eliminate or minimize the effects of these hypothecated factors, especially when compared to disease. 

If one wants to argue causation, then it is necessary to come up with a plausible mechanism for the correlated effects to become manifest.  The researchers note that intestinal worms have been shown to have an affect on cognition.  Malaria also has negative impacts on the brain; a study in Kenya showed that 1 in 8 children impacted by celebral malaria had long-term cognitive damage.  The researchers view the causes of diarrhea as the biggest threat.  It accounts for 1/6 of all infant deaths; those that do not die suffer from low absorption of nutrients during a key time when the brain is growing.

The hypothesis put forth in this research may also explain the "Flynn effect", which is a rise in intelligence over decades in rich countries.  Until now, the effect has escaped reasonable explanation.  The disease hypothesis described above provides a mechanism for that to occur. 

If the thesis holds up, then the elimination of disease should be a main aim of development, not an afterthought.  Vaccination, clean water, and sanitation now can be seen as more important than ever.

The study can be found at http://rspb.royalsocietypublishing.org/content/early/2010/06/29/rspb.2010.0973.