And the lead poisoning theory is amusing, although the degradation of the educational system and the media probably have more to do with it.
Well, here's why I started seriously thinking about the political impacts of lead poisoning.
It's basically been proven that lead poisoning is the cause of all of the crime waves in the 20th century. A lot of people don't want to admit it, but the evidence is overwhelming for a social science topic.
Read this, then read the linked studies:
http://www.motherjones.com/environment/2013/01/lead-crime-link-gasoline
Then read the followup articles (in no particular order since I've had trouble finding all of them):
http://www.motherjones.com/kevin-drum/2014/02/lead-and-crime-its-brain-thing
http://www.motherjones.com/kevin-drum/2015/01/yep-gasoline-lead-explains-crime-decline-canada-too
http://www.motherjones.com/kevin-drum/2014/09/prison-rates-are-down-thanks-lead-theyre-going-stay-down
http://www.motherjones.com/kevin-drum/2015/02/lead-and-crime-brennan-center-weighs
http://www.motherjones.com/kevin-drum/2014/08/childhood-lead-exposure-causes-lot-more-just-rise-violent-crime
http://www.motherjones.com/kevin-drum/2015/01/lead-and-crime-some-new-evidence-century-ago
To be clear, the link is between *childhood* lead exposure and violent crime rates *when those children become young adults*, so there's a ~18 year lag. Changes in lead presence in the environment, of course, lag the introduction or removal of lead in gasoline, paint, or pipes.
So, anyway we know from direct neurological studies that childhood lead poisoning causes lowered ability to reason and lowered inhibitions. (There are additional effects which are documented in adults, but it's more complicated.) We know that there was massive environmental exposure during a particular time period. We've already linked this to the crime rates among young adults.
But what happens when that same cohort reaches an older age? You'd *expect* them to be less able to reason and to be more prone to irrational violence. I take the theory quite seriously at this point.
It's worth noting that one of the main remaining sources of lead in the environment is from lead bullets. They volatilize and are breathed in at gun ranges (and people who shoot a lot at ranges frequently have elevated blood levels), while there are dangerous amounts in meat which is shot with lead bullets (there's a study from Iowa about this as well as a couple of others). As far as I'm aware I'm the only one who's suggested this, but if you've wondered why the NRA doesn't always seem to be exactly representing gun owners in a sane manner... maybe there's a reason for it.
People don't generally like to believe in the environmental-exposure explanations for social problems, but they've had some really large explanatory successes, often better than the "cultural" or the "genetic" explanations.