The reason, according to the Montreal-born psychologist Steven Pinker’s massive book with that title, is, above all, the civilizing effect of governments and cities.
From Elizabeth Kolbert’s review in the New Yorker (October 3): “The homicide rate in New Orleans last year was 49 per 100,000, roughly what Amsterdam’s was six hundred years ago. St. Louis’ murder rate in 2010 was about 40 per 100,000, around the rate of London in the fourteenth century.”
According to Steven Pinker’s book, continuing racism in the American South has made low-income blacks “effectively stateless.” Therefore, the whole region is several steps behind the Northeast, to say nothing of Europe.
Kolbert: “If, fifty years ago, someone had predicted that the Soviet Union would dissolve peacefully, that the Europeans would adopt a common currency, and that a reunified Germany would terrify no one, that person would have been viewed as a kook….”
On the relative calm of the last half-century in Europe, Steven Pinker quotes Winston Churchill, who said in 1955:
“It may well be that we shall by a process of sublime irony have reached a stage in this story where safety will be the sturdy child of terror and survival the twin brother of annihilation.”
Eric Koch’s book, The Weimar Triangle, is available at Indigo-Chapters and in your local bookstore. 
Entirely aside of whether a reunified Germany is frightening (that’s a subject addressed elsewhere), Churchill’s pithy conjecture deserves a caveat: the parent (of safety) and the twin brother (of survival) are, I fear, alive and well and waiting in the wings to see whether the script will bring on their turn again.
If the “better angels” now hold the stage, how lucky we are!
This blog says we are indeed lucky: the “better angels” are in charge.
Even in Germany.
It may be true that violence has declined in a part or parts of the world but is there persuasive evidence that world-wide violence has declined?
I think Pinker says it has.
Try this review in the NYTimes Book Review: http://www.nytimes.com/2011/10/09/books/review/the-better-angels-of-our-nature-by-steven-pinker-book-review.html?_r=2&ref=review
Levitt’s Freakonomics and Super Freakonomics provide very convincing metrics of the decline century over century, even in the US. Pinker likely drew from Levitt’s source docs. US started as a more violent society. Levitt points out that the surge during the post-war was an anomaly that has passed. However, global media attention to “local” crime (making a crime a continent away “feel” next door), and lower tolerance of violence leave people feel less safe, even if they are statistically exponential safer from violent crime than a century ago. A favorite example of mine is the demise of the bank robber with a gun. Credit card and ATM technology really destroyed that illicit “profession”, and “denied” most Canadians of the previous generations most frequent exposure to gun violence.
Mike Sky
What about all these suicide attacks in Afghanistan and Iraq ? Don’t they count ?
Now is the best of times and the worst of times, but overall, there’s more enlightenment, less tolerance for abuse of the weak by the powerful, and an awareness that the male propensity for domination and control is hard-wired, so we all have to be vigilant, personally, institutionally, nationally and globally, to ensure that men channel violent and controlling tendencies into constructive behaviour and collaborative action. Some of the shift towards peace and harmony comes from fear, but also from revulsion when we see our own species (still) behaving brutally.