From Discover magazine comes this fascinating article on iodized salt in the US and how it may have added IQ points to the population:
Iodine deficiency today is the leading cause of preventable mental retardation in the world. It’s estimated that nearly one-third of the world’s population has a diet with too little iodine in it, and the problem isn’t limited to developing countries—perhaps one-fifth of those cases are in Europe (pdf), where iodized salt is still not the norm...It's always dangerous to draw conclusions about causative relationships, but this is one of the most fascinating articles I've read in some time. It's enough to make me want to switch back from kosher salt.
The economists found that in the lowest-iodine areas—the bottom quarter of the study population—the introduction of iodized salt had stark effects. Men from these regions born in 1924 or later were significantly more likely to get into the Air Force and had an average IQ that was 15 points higher than their predecessors.