Mike LaBossiere expands on Pinker's thesis:
A rather interesting factor to consider is natural selection. Societies tend to respond to violent crimes with violence, often killing such criminals. Wars also tend to kill the violent. As such, centuries of war and violent crime might be performing natural selection on the human species-the more violent humans would tend to be killed, thus leaving those less prone to crime and violence to reproduce more. Crudely put, perhaps we are killing our way towards peace.
In an unrelated post, Pinker explores religion's role in the decreased violence. Earlier posts on Pinker's book here, here, here, here, here, here, here, here, here and here.
The Dish 