For a while now, Kevin Drum has been putting forth a really remarkable theory regarding lead levels in the environment (from gasoline and paint) and how that correlated with a rise and fall in crime rates. He's now helpfully compiled a sort of link-index to his writing on the subject. I really recommend at least reading through the first (long) link he's got there.

If you accept the theory (and the evidence seems strong) it's really humbling to realize how much we are products of our environment. And for more on that see this little blurb from Atrios which may sound snarky but really isn't.