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.