Disorder is a dauntingly vast topic. So, we are lucky that Pinker, a Harvard cognitive scientist whose 2002 work The Blank Slate may have been the outstanding book of the last decade, has turned his abundant energy and intelligence to understanding violence. No reductionist, Pinker attributes what he sees as the slow retreat from violence to "six trends" interacting with "five inner demons," "four better angels," and "five historical forces."
These 20 factors -- ranging from the rise of Leviathan to the expansion of empathy and rationality -- aren't really enough to explain trends in violence, but they're certainly a start. And I can't think of anybody who could have done a better job. Pinker's range is extraordinary. For instance, The Better Angels of Our Nature includes the best introduction to brain anatomy that I've read. (And Pinker isn't even all that terribly impressed by fashionable fMRI scans.) Yet, his touch is light. He sums up the research on why marriage makes men behave better with Johnny Cash's definitive explanatory couplet: "Because you're mine, I walk the line."
(And, in case you are wondering, yes, Pinker eventually does quote Edwin Starr's 1970 Motown hit single: "War! Huh, yeah, what is it good for?" Being Pinker, he presents a long list of the pragmatic uses of war, while remaining in emotional harmony with Starr's sentiment: "Absolutely nothing!")