The problem with the lead-crime hypothesis is that the cause of lead and the effect of increased crime are 'too far apart' with many intervening links in the causal chain, to paraphrase Steven Pinker [1]. He calls the hypothesis 'provocative, but far from proven.' The intervening links in the causal chain involve a lot of sub-causal links and hypotheses that are as yet, unproven.
Even if lead can't be directly related to crime, lead poisoning causes many bad things to the human body, including learning disabilities, and reducing adult brain size in children exposed.
[1] http://stevenpinker.com/files/pinker/files/pinker_comments_o...