"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."
- Ikhide
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