Mens sana in corpore sano

Parasites and pathogens may explain why people in some parts of the world are cleverer than those in others


The Economist
Jul 1st 2010

HUMAN intelligence is puzzling. It is higher, on average, in some places than in others. And it seems to have been rising in recent decades. Why these two things should be true is controversial. This week, though, a group of researchers at the University of New Mexico propose the same explanation for both: the effect of infectious disease. If they are right, it suggests that the control of such diseases is crucial to a country’s development in a way that had not been appreciated before. Places that harbour a lot of parasites and pathogens not only suffer the debilitating effects of disease on their workforces, but also have their human capital eroded, child by child, from birth.

Christopher Eppig and his colleagues make their suggestion in the Proceedings of the Royal Society. They note that the brains of newly born children require 87% of those children’s metabolic energy. In five-year-olds the figure is still 44% and even in adults the brain—a mere 2% of the body’s weight—consumes about a quarter of the body’s energy. Any competition for this energy is likely to damage the brain’s development, and parasites and pathogens compete for it in several ways. Some feed on the host’s tissue directly, or hijack its molecular machinery to reproduce. Some, particularly those that live in the gut, stop their host absorbing food. And all provoke the host’s immune system into activity, which diverts resources from other things.

The inverse correlation that the group calculated between a country’s disease burden and the average intelligence of its people is impressive. They estimated the disease burden from World Health Organisation data on DALYs (disability-adjusted life years) lost caused by 28 infectious diseases. These data exist for 192 countries. The intelligence scores came from work carried out earlier this decade by Richard Lynn, a British psychologist, and Tatu Vanhanen, a Finnish political scientist, who analysed IQ studies from 113 countries, and from subsequent work by Jelte Wicherts, a Dutch psychologist.

At the bottom of the average-intelligence list is Equatorial Guinea, followed by St Lucia. Cameroon, Mozambique and Gabon tie at third from bottom. These countries also have among the highest burden of infectious diseases. At the top of the list of countries with the highest average intelligence is Singapore, followed by South Korea. China and Japan tie in third place. These countries all have relatively low levels of disease. America, Britain and a number of European countries, follow behind the leaders. A list of the countries included in the study can be found here.

(More here.)