In a report published in the National Bureau of Economic Research, James Freyer, David Weil and Dimitra Politi examined data from about two million enlistees for World War II born between 1921 and 1927, comparing the intelligence levels of those born just before 1924 and those born just after.
To do this, they looked to standardized IQ tests that each recruit took as a part of the enlistment process.
While the researchers didn't have access to the test scores themselves, they had another way of gauging intelligence levels: smarter recruits were sent to the Air Forces, while the less intelligent ones were assigned to the Ground Forces.
Next, the economists worked out likely iodine levels in different cities and towns around America using statistics gathered after World War I on the occurrence of goiter.
Matching the recruits with their hometowns showed researchers that the men from low-iodine areas made a huge leap in IQ after the introduction of iodine.
The men born in low-iodine areas after 1924 were much more likely to get into the Air Force and had an average IQ that was 15 points above that of their slightly older comrades.
This averages out to a 3.5 point rise in IQ levels across the nation.[How adding iodine to salt made America smarter, By Alex Greig, DailyMail, July 24, 2013]
You can see something of a Flynn Effect in the historical record of conscription in the two world wars. American elites were shocked by what a sizable fraction of draftees in WWI were illiterate and/or kind of slow. WWII draftees were somewhat more satisfactory to the big shots.
In particular, mechanical skills were very good. Thanks to Henry Ford, et al, young American men entered the military in WWII knowing more about repairing internal combustion engines than any other country's soldiers. In contrast, the Japanese started the war in the Pacific with elite technicians capable of bombing Pearl Harbor, but they couldn't ramp up the numbers anywhere near as fast
But, I hadn't been aware of this 1924 bump.
Kiwanis International is the main charitable supporter of iodizing salt in poor countries.