Has violence actually declined over the past millennium?

When you consider that the late, great 20th century gave us such bellweathers of violence as two world wars and a holocaust, not to mention the atom bomb, this certainly is counter-intuitive.
But this is precisely what the Freakonconomics guy claimed on American Public Radio's Marketplace show tonight: http://marketplace.publicradio.org/display/web/2011/10/04/pm-freakonomics-crime/

The claim was based on a new book by a Harvard shrink: http://www.amazon.com/s?ie=UTF8&keywords=0670022950&tag=mktpl-20&index=blended&link_code=qs

Image: digitalart / FreeDigitalPhotos.net

For more on this, you can visit the Freakonomics website. For instance, has Internet porn reduced the incidence of rape? Find out at: http://www.freakonomics.com/2011/08/04/porn-and-rape-the-debate-continues/
Clue: Probably yes.http://www.scientificamerican.com/article.cfm?id=the-sunny-side-of-smut