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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. A great deal of research has shown that average IQ varies around the world, both across nations and within them.

Higher IQ predicts a wide range of important factors, including better grades in school, a higher level of education, better health, better job performance, higher wages, and reduced risk of obesity. So having a better understanding of variations in intelligence might yield a greater understanding of these other issues as well.

In a study in 2010, it was found that, among all the factors that affect intelligence, infectious disease works as the best predictor of the bunch. A recent study by Christopher Hassall and Thomas Sherratt repeated the study using more sophisticated statistical methods, and concluded that infectious disease may be the only really important predictor of average national IQ.

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.

Despite the strength of the findings, the study was not without its limitations. The researchers did their best to control for the effects of education. But what they really needed was to repeat their analysis across regions within a single nation, preferably one with standardized, compulsory education. The nation they chose was the United States. Average IQ varies in the states. Again, infectious disease was an excellent predictor of average state IQ. The states with the five lowest average IQ all have higher levels of infectious disease than the states with the five highest average IQ, and the relationship was good across all of the states in between.

So far, the evidence suggests that infectious disease is a primary cause of the global variation in human intelligence. Since this is a developmental cause, rather than a genetic one, it's good news for anyone who is interested in reducing global inequality associated with IQ. It will allow people interested in using this information to raise the IQ of people around the world to target their efforts most effectively and efficiently.

16. We can learn from the first paragraph that______.

[A] energy is the most important factor that affects intelligence

[B] newborn babies spend more calories than adults in running their brains

[C] extra energy cost may cause intellectual damage to children

[D] there are IQ variations among and within nations

17. Higher average IQ in one place may______.

[A] explain why people there can get a better understanding

[B] indicate the possibility of having an infectious disease

[C] show the history of less infectious diseases there

[D] show that people there are slimmer than those with lower IQ

18. The word "mutable" (Line 5, Para. 4) is closest in meaning to______.

[A] changeable

[B] exchangeable

[C] acceptable

[D] susceptible

19. Which of the following is true according to Paragraphs 4 and 5?

[A] Children with higher IQ may have suffered from intestinal worms.

[B] Malaria affects most Mexicans' physical health.

[C] Average IQ variation in the U.S. is not so obvious.

[D] Education is also a factor that affects intelligence.

20. The finding of the researches is beneficial in that______.

[A] it gives people the hope of eliminating social inequality

[B] it enables people to raise the IQ levels of mankind in a better way

[C] it helps to improve people's intelligence genetically

[D] it reduces the possibility of people's suffering from diseases