Being smart is the most expensive thing we do. Not in terms of money, but in a currency that is vital to all living things: energy. One study found that newborn humans spend close to 90 percent of their calories on building and running their brains. (Even as adults, our brains consume as much as a quarter of our energy.) If, during childhood, when the brain is being built, some unexpected energy cost comes along, the brain will suffer. Infectious disease is a factor that may rob large amounts of energy away from a developing brain. This was our hypothesis, anyway, when my colleagues, Corey Fincher and Randy Thornhill, and I published a paper on the global diversity of human intelligence.
A great deal of research has shown that average IQ varies around the world, both across nations and within them. The cause of this variation has been of great interest to scientists for many years. At the heart of this debate is whether these differences are due to genetics, environment or both.
Support for this hypothesis comes not only from cross-national studies, but from studies of individuals. There have been many studies, for example, showing that children infected with intestinal worms have lower IQ later in life. Another study by Atheendar Venkataramani found that regions in Mexico that were the target of malaria eradication programs had higher average IQ than those that were not. In practical terms, however, this means that human intelligence is mutable. If differences in IQ across the world are largely due to exposure to infectious disease during childhood, then reducing exposure to disease should increase IQ.
Because brain development depends highly on energy, nutrition and health are preconditions for smartness. One more example that the world is not fair!