Answer: Iodized Salt

When pondering what could cause entire swathes of the country to enjoy significant IQ gains, most of us would turn our attention to things that are inherently cerebral in nature: better-funded grade schools, literacy programs, or college tuition initiatives, for example.

One of the most significant impacts on the IQ levels of U.S. citizens, however, was not standardization of schooling, increased college attendance, or literacy programs, but the introduction of iodine-enriched salt. Starting in 1924, all non-kosher table salt in the U.S. was enriched with iodine, a micronutrient that is incredibly important to neurological development. Not only do children born from mothers who are iodine deficient have lowered cognitive functioning (damage, unfortunately, that cannot be undone later), but iodine deficiency is also the leading cause of preventable mental retardation worldwide.

So how big of an impact did the introduction of iodized salt have on IQ levels? Areas of the country with no natural sources of iodine (e.g. inland areas where iodine-rich seafood was rarely consumed) saw an average rise of 15 points over the following decade—a rise of one full deviation on the scale in just ten years.

The lessons of the early 20th-century experiment in iodizing salt aren’t lost on us today. Not only do we continue to add iodine to salt in the United States, but Unicef has led initiatives around the world to educate the public about the benefits of iodized salt in landlocked countries in order to bring the benefits of the micronutrient to children there too.