The lead/crime theory seems to have entered blogospheric conversation-space thanks to an article by Mother Jones writer/blogger Kevin Drum. Here's a link to most of what he's written on the subject in the past month. Here's an excerpt from the short version/intro to the magazine article:

The chart [above] illustrates the basic data that inspired the lead hypothesis: it shows lead emissions starting in 1935 overlaid with the violent crime rate 23 years later. The two curves match almost perfectly.

Now, I know my readers, and first thing a lot of you are going to do is yell at me: "Correlation is not causation!" And that's true. If this curve were the only bit of evidence we had, the connection between lead and violent crime would be pretty thin. But it's not. You should read the story to understand just how many different studies confirm this relationship...

We now have a huge amount of evidence linking lead to violent crime. We have evidence not just at the national level, but also at the state level, the city level, and the international level. We have longitudinal studies that track children from birth to adulthood to find out if higher blood lead levels lead to more arrests for violent crimes. And perhaps most important, this is a theory that just makes sense. Everything we now know about the effects of lead on the brain tells us that even moderately high levels of lead exposure are associated with aggressivity, impulsivity, ADHD, and lower IQ. And right there, you've practically defined the profile of a violent young offender.