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Violent crime rates matched lead emissions into the air 23 years prior almost exactly until the Bush years, when violent crime rates began to decline much more slowly than airborne lead levels.

Still, the correlation was close enough for long enough to infer at least some causality - especially given what we know about the neurotoxicity of lead on growing children and the harm it inflicts on their cognitive capabilities (including self-control faculties).

http://nextbigfuture.com/2013/01/strong-case-that-childhood-lead-leads.html

Baby Boom started hitting high crime life cycle stage. Relatively few people turn to a life of crime in their 40s after a blameless youth. Young men are the most likely killers. More young men as a % of the overall population, higher murder rate. Lead is interesting as well.