Mind & Body

How Iodized Salt (Accidentally) Increased the American IQ

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Sometimes a solution to one problem turns out to solve a problem you didn't even know you had. That's especially true when the problem is a nutrient deficiency. In the 1920s, the addition of iodine to our table salt had a brain-boosting side effect.

Improving SA(L)T Scores

In the 1920s, the United States had a serious goiter problem. Goiter is painful swelling of the thyroid gland that often results in a large, visible bulge on the neck and throat. It's a very unpleasant condition, and its chief cause is iodine deficiency. Before companies began iodizing salt, the amount of iodine in your diet would have been determined almost entirely by where you live. Live by the sea? Great news: you're practically swimming in the stuff. But in the Midwest, prehistoric glaciers leached the iodine out of the soil many years ago.

In an effort to fight the growing "goiter belt" — that's seriously what they called places without a lot of iodine back in the day — salt manufacturers in the United States began adding iodine to their table salts. And the goiter problem was solved almost overnight. But something else changed, too. According to a report released in 2013, the average IQ of people in iodine-poor areas increased by 15 points as a direct result.

Iodine on Your Mind

Working on a hunch that intellectual disabilities could be tied to iodine deficiency, James Feyrer, David Weil, and Dimitra Politi examined military records of recruits born in the 1920s. They weren't able to examine the actual test scores, but they found a workaround. Back then, the recruits with higher scores were sent to the Air Force and the ones with lower scores joined the ground forces. By comparing WWI-era goiter rates and test results with those from WWII, they could see a clear connection between a reliable source of iodine and a rising average IQ.

Curious about a different take on how we measure intelligence? Check out "IQ: A Smart History of a Failed Idea" by Stephen Murdoch, an unflinching look at the sometimes controversial history of testing smarts. We handpick reading recommendations we think you may like. If you choose to make a purchase through that link, Curiosity will get a share of the sale.

Why Is Iodine Added to Salt?

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Written by Reuben Westmaas February 21, 2018