With the Flynn effect at the fore of the blogosphere, I wonder if the reduction in the common use of lead in paint, gasoline, candles, sulder, and the like offers any explanation. It seems initially plausible. Richard Nevin, who also sees lead reduction as an explanation for a decrease in the US homicide rate, is the only person I've found to explicitly make the connection between the two. Are there others?
Lead is now understood to be harmful in the Western world (not so much the case yet in China) and has subsequently been phased out of use for most household products, especially those geared toward children. Doubling the lead level considered below the "level of concern" (although the idea of any safe level is in question) in the blood of children has been found to lead (sorry) to a drop of around six points in IQ.
Techniques for measuring lead levels in the blood prior to the sixties are not very reliable, but public awareness of its potentially harmful effects extend back to at least the 1930s (the first recognition of lead toxicity traces back as far 4,000 years ago). As the dangers became known, the use of lead was reduced. Because lead is abundant, malleable, and resistant to corrosion, it was not phased out as quickly and fully in lower-scale end-use products. Zinc-based replacements are often more expensive (something I find odd, as lead is actually slightly more expensive than zinc on the metals market--presumably it has to do with manufacturing costs, since lead is so easy to use). So the latent removal of lead among products consumed by less affluent people might figure into why the Flynn effect has primarily occured on the lower-end of the intelligence distribution.
The deceleration of the Flynn effect in the developed world also meshes well with the plauisibility of the lead culprit. There is evidence that even trace amounts of lead in the bloodstream have a deleterious effect on IQ, but the benefits of reducing exposure further must be facing diminishing returns.
Might the liberal use of lead in China also partially explain that country's relatively modest average IQ compared to other East Asian nations like Taiwan, Hong Kong, Japan, and South Korea?