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.
Interesting. I wonder how much is causation and how much is correlation, however. People of lower socio-economic status would be less likely to afford deleading of their homes, or less inclined to make a stink about lead paint to their landlords. (In MA, a homeowner is required to delead any apartment where a child will be living. I've known people who broke their lease because their landlord figured it would be cheaper to let them move and have the apartment sit empty for a few months than to delead the apartment for a new baby.) It's clear that lead poisoning affects the brain, but it seems that this must be just one of many negative factors affecting the Pittsburgh teens who were arrested.
Laura - If you are at all interested or curious, there is some really fascinating research on lead and criminal behavior. Some was on NPR just yesterday, for example.
As far as I have seen (being only an amature), the evidence is starting to pile up on the side of causation, not correlation, and the statistics are downright shocking (i.e. lead has an amazingly strong effect on bad behavior).
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.
I assume this means that such a drop in IQ can be caused even by levels of lead currently considered "safe" - and thus that the guidelines ought to be made stricter.
Great post. I just picked up the lead discussion here:
Fascinating as well is the correlation, not coincidence, between criminal behavior and IQ. Children born to mothers with poor diet, drug habits and now lead exposure, then raised in environments with poor diet, drug exposure and lead are far more likely to become dangerous due to pre frontal and epecially anterior cingulate damage which also correlates with low IQ. We really should champion brain health in general but especially frontal cortex health as a means to a safer and less coslty (in terms of crime and health issues) society. I am glad to see this getting much deserved attention. Its staggering to think how many violent crimes have been committed b/c of brain damage in early pre and post natal development.