Still more correlations...

The NBER, of whom I can be suspicious at times, has an interesting paper out:

http://papers.nber.org/papers/w13097

Environmental Policy as Social Policy? The Impact of Childhood Lead Exposure on Crime
Jessica Wolpaw Reyes NBER Working Paper No. 13097
Issued in May 2007

Abstract : Childhood lead exposure can lead to psychological deficits that are strongly associated with aggressive and criminal behavior. In the late 1970s in the United States, lead was removed from gasoline under the Clean Air Act. Using the sharp state-specific reductions in lead exposure resulting from this removal, this article finds that the reduction in childhood lead exposure in the late 1970s and early 1980s is responsible for significant declines in violent crime in the 1990s, and may cause further declines into the future. The elasticity of violent crime with respect to lead is estimated to be approximately 0.8.

If it is true, even partially, let's put one more nail in the coffin of "tough" law enforcement and high-incarceration rates of non-violent criminals as the cause for dropping crime rates. Like so many things, crime is a complex system with phyical and social environmental factors. Three-strike laws are easy sells, but they are hard to prove effective.