"Believe it or not—and I know most people do not—violence has been in decline over long stretches of time, and we may be living in the most peaceful time in our species' existence. The decline of violence, to be sure, has not been steady; it has not brought violence down to zero (to put it mildly); and it is not guaranteed to continue. But I hope to convince you that it's a persistent historical development, visible on scales from millennia to years, from the waging of wars and perpetration of genocides to the spanking of children and the treatment of animals." —Steven Pinker

Quote from a later speech. There is also a book.