We know that infectious disease inflicts many burdens on developing countries -- high infant mortality rates, for instance, and shortened lifespans. But new research highlighted in the Economist this past week indicates that perhaps too many pathogens can result in lower intelligence among the populace, too. You can read the article here.
While controversial, here is the explanation. In young children, a developing brain demands the majority of a body's metabolic energy. Any competition for this energy will result in less energy for the brain at a crucial juncture. Infectious diseases require quite a bit of energy to fight (or result in diarrhea, which prevents the body from absorbing nutrients which would feed the brain). Some practical evidence seems to support this; many children who manage to recover from cerebral malaria suffer cognitive disabilities afterward.
Of course, one has to be careful with all this. For some time, "experts" cautioned that teen girls shouldn't be educated because tending their brains would divert energy from their developing reproductive systems -- a sort of reverse of this hypothesis. But it does appear that the countries with the highest measured average intelligences (like Singapore and South Korea) have the lowest disease burdens. And, of course, this explanation does offer hope for future development. If you eradicate disease, you get a human capital boost apart from the fact that people will be healthier. They may be smarter too.
And as readers of this blog know, intelligence matters. A 150 IQ kid whose intelligence is lost due to disease is an asset a developing country will sorely miss.