The widespread addition of iodine to salt beginning in 1924
increased IQ in America by as much as 15 points, p
revious research has found.Along with improved health outcomes, adding iodine to salt also helped boost the U.S. economy.
For the generation that followed those born pre-iodization, labor force participation increased by 1% and high school completion rates jumped by 2%, according to a new paper from the University of Michigan’s Achyuta Adhvaryu and his co-authors.
The boost was higher in areas that had particularly high levels of iodine deficiency, and even helped spur the surge in female labour participation in the mid 20th century, as the effects were concentrated among women, according to the paper.
Other factors, of course, from growing societal acceptance of women working outside the home to a booming post-war economy played a part as well, but particularly in areas where iodine deficiency and related disorders were common, the effect was substantial.
The numbers may seem small at first glance, but they’re significant. And it’s fascinating that something as simple as fortifying a common ingredient could change people’s lives inside of a decade by dramatically reducing iodine deficiency and related diseases.
Incredibly, this is one of the few major public health interventions that was carried out entirely by the private sector. In fact, it was basically one company, the Morton Salt Company, that did the heavy lifting. Iodization was never legally mandated, and it cost the taxpayer almost nothing.
The IQ boost, and the likely source of most of the economic gain, come from the fact that children born to iodine-deficient mothers can have significant and irreversible cognitive impairment. It remains the largest preventable cause of mental retardation in the world. Women are more sensitive to iodine deficiency, and see more significant cognitive benefits from its consumption, according to the paper.
Iodization is powerful for two reasons. First, iodine deficiency is highly localised, most often seen far away from the sea in mountainous or glaciated areas, where erosion has reduced the iodine in the topsoil, as these maps show. The black areas on the left map show areas of high goiter occurrence, and the shaded areas on the left show places where there are low levels of iodine in the drinking water:
Second, iodization happened incredibly rapidly, and only a small amount is required to prevent deficiency. After Morton introduced iodized salt nationwide in 1924, the U.S. went from near-zero to near-universal availability in just half a decade. Iodine deficiency rates plummeted dramatically and rapidly in areas where it had been high.
That made the effect on “positive wage earning,” defined as earning more than one received in government assistance, and labour force participation particularly dramatic in high-deficiency areas.
The first chart below shows labour force participation in high goiter areas (indicating iodine deficiency) before and after iodized salt’s introduction compared to low goiter areas. The below chart does the same for positive earnings:
As big and rapid an effect as iodization had in the U.S., nearly a third of the world still doesn’t have sufficient access to iodine. There’s still a great deal of work to be done.
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