Here's a link to a nice overview of the lead hypothesis (with references and links): article about the lead hypothesis.  There's also a discussion of it in the Cosmos episode "The Clean Room" which you can stream on Netflix (it's episode 7 of the new series, not the old Carl Sagan one).  This also looks relevant, although I haven't seen it: documentary on lead.  Interestingly, that film apparently makes the case that lead is still extremely pernicious, for reasons quite independent of violent crime.  Here's a blurb from the "About the Film" section of the site to which I just linked:

The question on many minds today: what is the source of the sudden, alarming rise in the number of American children with ADD, ADHD, Autism Spectrum symptoms and similar neurological disorders—expensive impairments/disabilities that create challenges for families and cost our society more than $50,000,000,000 annually?  [JT: presumably the answer the film defends is that lead has something to do with it.]