People who live in countries where disease is rife have lower IQs because they have to divert energy away from brain development to fight infections

The idea helps to explain why national IQ scores differ around the world, and are lower in some warmer countries where debilitating parasites such as malaria are widespread. Researchers behind the theory point out that the impact of disease on IQ scores has been under-appreciated, and believe it ranks alongside education and wealth as a major factor that influences cognitive ability. The latest theory, put forward by Randy Thornhill and others at the University of New Mexico, adds disease to a long list of factors that play a role in determining intelligence. Thornhill made the news in 2000, when he coauthored a provocative book called A Natural History of Rape in which he argues that sexual coercion emerged as an evolutionary adaptation. Thornhill and his colleagues explain that children under five devote much of their energy to brain development. When the body has to fight infections, it may have to sacrifice brain development. To test the idea, Thornhill's group used three published surveys of global IQ scores and compared them with data from the World Health Organization (WHO) on how badly infectious diseases affect different countries. The list included common infections, such as malaria, tetanus and tuberculosis. The scientists found that the level of infectious disease in a country was closely linked to the average national IQ. The heavier the burden of disease, the lower the nation's IQ scores. Thornhill believes that nations who have lived with diseases for long periods may have adapted, by developing better immune systems at the expense of brain function. The effect of infectious disease on IQ is bigger than any other single factor. Disease is a major sap on the body's energy, and the brain takes a lot of energy to build. If you don't have enough, you can't do it properly. The consequence of this is that the IQ of a nation will be largely unaffected until you can lift the burden of disease. The finding explains about 50% - 60% of the variability in IQ scores and appears to be independent of some other factors such as overall GDP. For reasons that are unclear, IQ scores are generally rising around the world. Thornhill suggests monitoring rates of infectious diseases in nations as they develop, to see if they decline and IQ tests scores rise. Richard Lynn, professor of psychology at Ulster University, and author of the 2002 book, IQ and the Wealth of Nations, said disease and IQ is a two-way relationship, with low national IQs being partly responsible for widespread infectious diseases. In recent decades, HIV has been a serious infectious disease, and it has a high infection rate in low IQ countries, especially in southern Africa, where it is present in around 30% of the population. This is attributable to the low IQ of the population who do not understand the way the infection is contracted, and have erroneous beliefs about how to prevent infection.