Africa, parasites, intelligence To understand intelligence in the tropics, e.g. among dark-skinned Africans, you need to understand the importance of parasites—both as an environmental factor that exerts a negative effect on mental development and a positive effect on selection for biological diversity of all sorts, including intelligence. 1. Taking first the well-known negative effect of parasite load on individual intelligence—in the US the parallel factor is toxins and also indoor parasites, as in tenements—the US is now more segregated by race than it was in 1965 and so that many studies show that there is a strong tendency for whites to live in healthy environments and blacks inferior habitats. For Africa, the environmental factor is parasites—disease—known to exert a strong negative effect on intelligence.