Derb: In Constant Battles, a book about prehistoric warfare by Steven A. LeBlanc, we learn that violence was a surprisingly important demographic reality in early societies–something like 25 percent of all men could expect to lose their lives in battle. This figure appears to hold up across time and geography, applying even to Neanderthals whose bones have been studied forensically. For all of the fighting in Iraq and elsewhere, the world has actually never been more peaceful than it is right now.