Kevin Drum has an interesting article out on the spectacular drop in violent crime since the early 1990's and its possible connection with lead. Says the theory: as leaded gas was eliminated, there was a corresponding reduction of pathologies in young people.
However, throwing about 1% of the population in prison has a way of reducing crime. Having the largest incarceration rate in the world does have an affect, at a huge economic and human cost.
Part of that surplus labor Marx talked about is swept away, hiding the fact that capitalism is inherently incapable of providing full employment.