For many of the world's people, a small deficiency in iodine makes a big difference in IQ. Using iodized salt is standard in advanced countries, but in many places is looked upon with suspicion. Iodizing salt is one of the simplest, cheapest (a bit over $1 per ton) and most effective public health measures available. This New York Times story by Doug G. McNeil, jr. has the details:
Valentina Sivryukova knew her public service messages were hitting the mark when she heard how one Kazakh schoolboy called another stupid. “What are you,” he sneered, “iodine-deficient or something?”
Some more excerpts:
Worldwide, about two billion people — a third of the globe — get too little iodine, including hundreds of millions in India and China. Studies show that iodine deficiency is the leading preventable cause of mental retardation. Even moderate deficiency, especially in pregnant women and infants, lowers intelligence by 10 to 15 I.Q. points, shaving incalculable potential off a nation’s development...
In the 1990s, when the campaign for iodization began, the world’s greatest concentration of iodine-deficient countries was in the landlocked former Soviet republics of Central Asia.
All of them — Kazakhstan, Turkmenistan, Tajikistan, Uzbekistan, Kyrghzstan — saw their economies break down with the collapse of the Soviet Union. Across the region, only 28 percent of all households used iodized salt.
Naturally, there is resistance, usually on the basis of superstition or just because salt companies don't want to be bothered.