Nicholas Kristof has an important column on the link between iodized salt and IQ in developing countries:
Almost one-third of the world’s people don’t get enough iodine from food and water. The result in extreme cases is large goiters that swell their necks, or other obvious impairments such as dwarfism or cretinism. But far more common is mental slowness.
When a pregnant woman doesn’t have enough iodine in her body, her child may suffer irreversible brain damage and could have an I.Q. that is 10 to 15 points lower than it would otherwise be. An educated guess is that iodine deficiency results in a needless loss of more than 1 billion I.Q. points around the world.
A similar phenomenon exists here, in the United States, although our problem isn’t salt: it’s lead paint. A 2003 study in the New England Journal of Medicine found that blood levels of lead below current federal and international guidelines of 10 micrograms per deciliter produce a surprisingly large drop in IQ of up to 7.4 points. The researchers estimated that one in every 50 U.S. children has lead levels above that guideline. Seven points of IQ might not sound like a lot, but when you’re at the lower end of the IQ spectrum those points can be pretty crucial.
The initial evidence suggests that lead poisoning affects the brain by damaging the prefrontal cortex, thus leading to a loss of cognitive skills like self-control and working memory. There’s also some suggestive statistical evidence linking lead exposure and criminal activity. And then there’s this:
In 2002, Herbert Needleman, a psychiatrist at the University of Pittsburgh, compared lead levels of 194 adolescents arrested in Pittsburgh with lead levels of 146 high school adolescents: The arrested youths had lead levels that were four times higher.