Iodine deficiency is the leading cause of preventable mental retardation in
the world today. Iodine deficiency was common in the developed world until
the introduction of iodized salt in the 1920’s. The incidence of iodine deficiency
is connected to low iodine levels in the soil and water. We examine the impact
of salt iodization in the US by taking advantage of this natural geographic
variation. Areas with high pre-treatment levels of iodine deficiency provide
a treatment group which we can compare to a control group of low iodine
deficiency areas. In the US, salt was iodized over a very short period of time
around 1924. We use previously unused data collected during WWI and WWII
to compare outcomes of cohorts born before and after iodization, in localities
that were naturally poor and rich in iodine. We find evidence of the beneficial
effects of iodization on the cognitive abilities of the cohorts exposed to it
