Via Kevin Drum, here is a fascinating link from the Washington Post. Economist Rick Nevin has been able to draw a high degree of corelation between crime statistics and the lead levels of years before when the criminals were toddlers. Most impressively, the corelation seems to hold true in nine different countries with varying regulatory histories. This corelation is stronger and more consistent than other celebrated theories, such as the Freakonomics idea that the legalization of abortion was responsible for falling crime... or the Rudy idea that all credit belongs to Rudy.
Kevin uses the opportunity to agitate for lead abatement, pointing to research showing that a reduction in lead levels would improve IQs measurably.
Coincidentally, I just finished reading a book about Roman times -- Augustus by Anthony Everitt -- that repeatedly cites uses of lead that would shock us today: the metal was used in make up, water pipes, wine-making, weaponry, and even cooking pots. The ubiquity, moldability, and low melting point of lead made it very popular in ancient times. The sort of poisoning that must have come from such use would surely have dwarfed the contamination we saw in the 20th century. Could this explain the violent, unstable nature of Ancient Rome?
A google search shows that this is not an original thought. Different theorists have put lead poisoning forth as a possible cause of the Roman Empire's fall.
It's helpful to remember... societies are very capable of poisoning themselves to death.