Goiter, an enlargement of the thyroid gland, is caused by a lack of iodine in the diet. In the early 20th century, goiter was a significant public health problem in many countries, especially in areas far from the ocean where seafood, a natural source of iodine, was not readily available. To address this issue, iodine was added to table salt, which is widely consumed, and became a simple and effective way to ensure that people received the iodine they needed.
The effects of iodine fortification were quickly apparent. In countries where iodized salt was introduced, the incidence of goiter dropped dramatically, and studies have shown that the addition of iodine to salt has also led to a significant increase in IQ. One study involving Air Force recruits found that the average IQ of a population increased by 3.5 points after its introduction.
In the study, a group of economists observed a natural experiment by comparing the intelligence of children born before and after the year iodization began in 1924. The economists, James Freyer, David Weil, and Dimitra Politi, used military data from the early 1920s when millions of men and women enlisted during World War II.
As part of their enlistment, recruits took a standardized intelligence test. While researchers did not have access to the actual test scores, they used a clever substitute: smarter recruits were assigned to the Air Forces while less intelligent ones went to the Ground Forces. This allowed the researchers to infer test scores based on which branch a recruit was selected for.
Intelligence data was combined with birthdate and hometown information since iodine levels in soil and water vary greatly by location. To estimate which regions had high-iodine levels and which had low, the researchers referred to nationwide statistics collected after World War I on the prevalence of goiter.
The economists had sufficient data on approximately 2 million male recruits born between 1921 and 1927. In the lowest-iodine areas, which comprised the bottom quarter of the study population, the introduction of iodized salt had significant effects. Men from these regions born in 1924 or later were considerably more likely to join the Air Force and had an average IQ that was 15 points higher than their predecessors.
The addition of iodine to salt has become an essential public health measure, and many countries now require iodization of salt by law. The World Health Organization (WHO) estimates that approximately 2 billion people worldwide do not consume enough iodine, and iodine deficiency is still a significant public health problem in some parts of the world.