Discover Magazine reports:
With this background, then, a group of economists saw a natural experiment: comparing the intelligence of children born just before 1924—the year iodization began—and those born just after. James Freyer, David Weil and Dimitra Politi used military data from the early 1920s, when World War II drove millions of men and women to enlist.
Recruits all took a standardized intelligence test as part of their enlistment. Researchers didn’t have access to the test scores themselves, but they had a clever substitute: smarter recruits were assigned to the Air Forces while the less bright ones went to the Ground Forces. This allowed the researchers to infer test scores depending on which branch a recruit was selected for.
Intelligence data were paired with birthdate and hometown, since iodine levels in the soil and water vary significantly from place to place. To estimate which regions were naturally high-iodine and which were low, the researchers referred to nationwide statistics collected after World War I on the prevalence of goiter.