Iodization of Salt Responsible for Increase in IQ

The researchers based the study on the Army General Classification Test (AGCT) of people who were born during the time period (1920–1927) when iodized salt was first introduced, interpreting data of US Air Force enlistments, which received those who scored significantly higher on the AGCT.

The researchers found that the number of men who scored well and went to the Air Force from low iodine areas increased by a full IQ standard deviation (15 points) after iodized salt was introduced. Since one quarter of the US population lived at that time in iodine-deficient areas, the result corresponds to a 3.5 point increase nationwide. Since IQs go up by about 3 points every decade—known as the Flynn effect—the iodization of salt may be responsible for a full decade's worth of increasing IQ in the US.

On the other hand, the researchers also documented a large increase in thyroid related deaths following the countrywide adoption of iodized salt, which affected mostly older individuals in localities with a natural high prevalence of iodine deficiency such as Michigan, where melting glaciers depleted iodine rich soil. The researchers also found that 0.5% of U.S. residents still suffered from serious goiter following salt iodization. The study was published as an NBER working paper in July 2013.

“Our results show that there were measurable cognitive benefits from iodizing salt that went beyond the obvious effect of reducing goiter in the US,” concluded study authors Professors James Feyrer, PhD, Dimitra Politi, PhD, and David N. Weil, PhD, of the NBER. “These results must be seen in the context of many, many other health interventions that were happening at the same time. Interventions of the magnitude of iodizing salt will generate a very large and noticeable effect on overall cognitive ability.”

Supplementation of iodine in salt, water, or oil increases body iodine levels and reduces deficiency disorders, with females benefiting more from iodization than males. The original waves of iodization worldwide caused large-scale changes in the number of people going to school, their occupation, and how they voted. A worrisome current trend is the developed “war on salt.” Existing iodized table salt often has far less iodide than recommended, while un-iodized salt or low-iodide salt like sea salt is ever more popular. Iodized salt used in cooking loses large chunks of its iodine content.

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