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Reviewing the records of two million recruits, Feyrer and his colleagues also checked the natural iodine levels in their hometowns. Nationwide, the researchers found, the introduction of iodine raised the average IQ by an estimated 3.5 points. And in the parts of the country where natural iodine levels were lowest, Feyrer and his colleagues estimated that scores leaped 15 points. It may be hard to believe that such a straightforward change in people’s diets could have such a tremendous effect on intelligence. But as public health workers continue to bring iodine to more of the world, the same jumps happen. In 1990, Robert DeLong, an expert on iodine at Duke University, traveled to the Taklamakan Desert in western China. The region has extremely low levels of iodine in the soil, and the people in the region have resisted attempts to introduce iodized salt. It didn’t help that the people of the region, the Uyghurs, distrusted the government in Beijing. Rumors spread that government-issued iodized salt had contraceptives in it, as a way to wipe out the community. DeLong and his Chinese medical colleagues approached local officials with a different idea: They would put iodine in the irrigation canals. Crops would absorb it in their water, and people in the Taklamakan region would eat it in their food. The officials agreed to the plan, and when DeLong later gave children from the region IQ tests, their average score jumped 16 points.
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Carl Zimmer (She Has Her Mother's Laugh: What Heredity Is, Is Not, and May Become)