A new 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.

In the absence of sources like iodized salt, a person's iodine intake is dictated largely by geological location. Much of the United States is home to iodine-depleted soil and water. Prior to 1924, when iodine was introduced into salt in the U.S., many people were iodine deficient.

Using test results from the Army General Classication Test (AGCT), collected 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 effect that translates to 15 I.Q. points.