I was skeptical, as this is the first time I've come across an article alleging a causal relationship between lead exposure and criminality, but I did a quick search for additional research and found that a study allegedly found lead-exposed counties had 400% the murder rate of low-lead counties, even after controlling for other environmental factors and socioeconomic differences.
However, this article doesn't address the costly "War on Drugs" and its contribution to prison populations, especially private, for-profit prisons that feed on heavy sentences for non-violent offenders. As the WoD becomes less popular, with two states voting to legalize marijuana in 2012, it follows that fewer youth would be newly incarcerated.
Nevertheless, the trend is remarkable.