Kevin Drum has an article in Mother Jones arguing that lead in the environment is the best explanation for the rise and fall of crime.  He states:

Put all this together and you have an astonishing body of evidence. We now have studies at the international level, the national level, the state level, the city level, and even the individual level. Groups of children have been followed from the womb to adulthood, and higher childhood blood lead levels are consistently associated with higher adult arrest rates for violent crimes. All of these studies tell the same story: Gasoline lead is responsible for a good share of the rise and fall of violent crime over the past half century.

It's an interesting theory and one I haven't heard before, but I remain skeptical of any singular theory regarding the cause of crime. Drum states that "more prisons might help control crime, more cops might help, and better policing might help. But the evidence is thin for any of these as the main cause." At this point, I don't think anyone can seriously discount the effect of incarceration on crime rates, even though it is not the exclusive explanatory variable in the mix.  Like all things in life, there are a variety of factors that likely account for the rise and fall of crime.  But putting recidivists behind bars is one of the big ones because despite calls to the contrary, the best predictor of future behavior really is past behavior.