A new working study published at the National Bureau of Economic Research concludes the iodization of salt in the United States in the early 1900s raised the I.Q. scores of some populations by as much as 15 points – a full standard deviation – in the span of just 10 years.
Photo by Marcos Ojeda, via flickr
"This salt supplies iodide," reads the container pictured above, "a necessary nutrient." Necessary, indeed. Iodine deficiency, which affects an estimated two-billion people on Earth, is widely recognized as the leading preventable cause of mental retardation worldwide.
Iodine is crucial to thyroid function, and thyroid function is crucial to a truckload of other biological processes. In the absence of iodine, your thyroid loses the ability to produce two iodine-containing hormones called T3 and T4. In adults, these thyroid hormones are used to regulate metabolism in the body, but they're also hugely important in neonatal brain development. If a pregnant mom is iodine deficient, her child can suffer cognitively.
You don't need much of the stuff to reap iodine's benefits; a ton of salt requires just two ounces of potassium iodate (KIO3) to qualify as iodized. At a little over fifty cents an ounce, KIO3 is a cheap, and hugely beneficial, supplement. A 2006 feature from the NYT (title: "In Raising the World's I.Q., the Secret's in the Salt") concludes that "putting iodine in salt... may be the simplest and most cost-effective health measure in the world."
The effects of iodine deficiency are keenly felt by children born to iodine-deficient mothers, and several studies point to the significant impact of iodization on intelligence and educational attainment. A 2009 study on the effects of iodine outreach in Tanzania, for example, found children exposed to iodine in the womb spent significantly more time in school than unexposed siblings and peers.
The most recent study to examine the impact of iodinizaion is presented in a working paper at the National Bureau of Economic Research. In the absence of sources like iodized salt, a person's iodine intake is dictated largely by her geological location. Seawater, for instance, is rich in iodine, so people in coastal areas are often exposed to the nutrient by eating seaweed, or food grown in iodine-rich soil. Much of the United States, however, is home to iodine-depleted soil and water; prior to 1924, when iodine was rapidly introduced to salt across the U.S., these are places whose residents struggled with iodine deficiency. Using test results from the Army General Classication Test (AGCT), collectured during the first and second World Wars, researchers James Feyrer, Dimitra Politi and David Weil compared the impact of iodization on populations living in regions that were naturally poor and rich in iodine.
"We find that for the one quarter of the population most deficient in iodine this intervention raised IQ by approximately one standard deviation," write the researchers, an affect that translates to 15 I.Q. points. "Our results can explain roughly one decade's worth of the upward trend in IQ in the U.S. (the Flynn Effect)."