Iodized salt is so ubiquitous that we barely notice it. Few people know why it even exists. Iodine deficiency remains the world's leading cause of preventable mental retardation. According to a new study, its introduction in America in 1924 had an effect so profound that it raised the country's IQ.

A new NBER working paper from James Feyrer, Dimitra Politi, and David N. Weil finds that the population in iodine-deficient areas saw IQs rise by a full standard deviation, which is 15 points, after iodized salt was introduced.

Since one quarter of the population lived in those areas, that corresponds to a 3.5 point increase nationwide. We've seen IQs go up by about 3 points every decade, something called the Flynn effect, so iodization of salt may be responsible for a full decade's worth of increasing IQ in the U.S.

If a mother is iodine deficient while she's pregnant, the cognitive development of the fetus is impeded, and the effects are irreversible. To this day, the World Health Organization estimates that nearly 50 million people suffer some kind of mental impairment related to iodine deficiency.

Before iodized salt, people were deficient based almost entirely on geography, whether the water and soil in their area had enough of the micronutrient. Diseases resulting from the deficiency, most commonly goiter, or swelling of the thyroid, were extremely common.

The differences by geography were vast, making the effects easy to isolate. Seawater, for example, is rich in iodine, but glaciers depleted iodine rich soil in places like Michigan:

The mental impacts were unknown, the program was started to fight goiter, so these effects were an extremely fortunate unintended side effect.

To figure out the effects, the researchers used the data from the Army General Classication Test (AGCT) given to people who enlisted during World War 2. That covers a wide group of men born precisely at the time iodized salt was introduced (1920-1927), which allowed comparison of low and high iodine areas.

The Air Force received enlistees who scored significantly higher on the AGCT, and the number of men who scored well and went to the Air Force from low iodine areas dramatically increased after iodized salt was introduced. The estimates of intelligence increases are based on that data.

Here's the author's chart that shows the boost in Air Force enlistment rates:

Despite these positive effects, there were some negative side effects as well. People who suffer long-term iodine deficiency can actually end up with from hyperthyroidism when it's introduced to their diet, so deaths spiked for a few years. However, the aggregate effect has been extremely positive.