Enter separate coincidental studies by several researchers on lead (the element--Pb) as a cause of violent behavior. Kevin Drum wrote a fantastic article for Mother Jones chronicling the discoveries:
"Put all this together and you have an astonishing body of evidence. We now have studies at the international level, the national level, the state level, the city level, and even the individual level. Groups of children have been followed from the womb to adulthood, and higher childhood blood lead levels are consistently associated with higher adult arrest rates for violent crimes. All of these studies tell the same story: Gasoline lead is responsible for a good share of the rise and fall of violent crime over the past half century."
Drum's article had a huge impact on how I think about the mind and human behavior. I thought about it when I was dealing with very rowdy, often violent children in the Turkish classroom. I think about it when I see adults on the news commit violent crimes that make no sense. "Sin nature" is all too often the simple catch-all that Christians give to describe such things. But the last 30 years haven't seen a decline in the sin nature of people around the world. A person's environment plays a major role in behavior and his ability to control violent urges. These studies seem to show that pretty clearly.
Drum is well left-of-center, and trumpets that it's government outlawing of leaded gasoline that plays the major role in reducing lead emissions. The market, left to its own devices, may have never banished the lead is the argument. I have no idea what the facts say on that, often industry is already eliminating harmful elements (see: child labor) before government gets involved.
But I highly recommend the article and its follow-ups just for the pure thought-provoking information about the effects of lead pollution on people and society.