Nassim Taleb, author of Fooled by Randomness and The Black Swan, criticizes Steven Pinker's book on the decline in violence, The Better Angels of Our Nature, in a short essay called The "Long Peace" Is a Statistical Illusion.
Pinker answers in Fooled by Belligerence.
To my mind, the scariest precedent isn't World War One, it's the American Civil War. Europe in 1914 was a militarized continent divided into language groups with various arms races and a lot of professional soldiers looking for a fight. It was a catastrophe, but you could kind of see it coming.
In contrast, most of the world seems to be demilitarizing today, with arms races becoming ever sleepier as inventiveness gets sidetracked into making cooler Powerpoints to boggle other laptop warriors.
In contrast to Europe in 1914, America in 1861 was a prosperous, peaceful, unmilitaristic country with one language, a pretty good political system for working out problems, and only a tiny group of professional soldiers, who had all they could handle fighting Indians out on the frontier. Indeed, a large fraction of the officers of 1860 spent 1861-1865 still out on the frontier, while retirees and amateurs won fame on the battlefields of the Civil War.
I wonder if the rise in organized sports since 1865 has lessened the chances of a dust-up by absorbing and re-directing communal excitements. The Civil War more or less made baseball the national pastime (it gave soldiers something to do in army camps). On TV today, you could see a lot of extremely excited people in towns like Palo Alto and Tuscaloosa cheering on their young heroes on the football field.
Or maybe the decline in family size?
These aren't original theories, of course. I wonder how you could test them?