I discovered an interesting article recently (several actually, but this one does a good job of summarizing them all, without getting too involved in the emotion of politics, as so many popular "scientific articles" seem to do of late.
The short story: a number of studies have turned up a very interesting, very strong correlation between childhood exposure to lead, and adult tendencies to violence.
In other words, there is a possibility that much of today's violence is the result of childhood exposure to leaded paint, leaded gasoline, and lead plumbing pipes. The suggestion is that childhood exposure to lead impacts normal brain development (there is evidence that it does effect IQ), in a way that causes an increased tendency for violence in later years (we've been trying to outlaw guns, when we should have been outlawing painters and plumbers).
In theory, that means we should expect a continued reduction in levels of violence in the US over the next decade or so, as the last of those who grew up in the leaded gasoline era pass away. Lead paint is already mostly gone in the US. Lead pipes are likely still a problem in older structures, particularly in Larger cities. The middle east appears to still be using the stuff, so they are likely to be mired in violence for the foreseeable future ...
The implications of this, if proven true are considerable. For instance many of the current autoimmune disorders (Crohn's, MS, IBS, etc...) are thought to have environmental triggers. Will a similar correlation be found for these, related to some other, now common substance?
The most notable point is the time span. This correlation was not discovered until decades after contamination occurred. Based on the observed correlation, the symptoms didn't manifest until roughly 20 years after contamination. This is a significant issue considering our relative lack of ability to effectively and accurately, acquire and retain sufficient quantities of data to really see the possible long-term correlations (In fact, a fairly significant effort goes into the hiding and/or destruction of old [often meaning older than three months] data, in an effort to protect against litigation, or preserve "state secrets").
We need to get better at collecting, managing, and understanding data.