The lead-violence correlation is very, very interesting. What always makes me wonder is how much historical effect environmental factors like this must have had. Lead used to be ubiquitous, between paint, pipes, roofing, glass, solder - the Romans even cooked in lead vessels at times.

There's probably no way of ever teasing out all the different factors, but when you read accounts of what daily life was like in, say 18th century London, and a.) violence was quite common-place, and b.) virtually everyone kept themselves inebriated enough to fail a breathalyzer at all times. Probably b.) contributed a good deal to a.), but if everybody is also suffering from low-level chronic lead poisoning...