Since 10,000 years ago , Stone Age says the risk of violent death has fallen to 10 to 20 % . In perhaps greatest paradox in history , what 's made the world safer is war itself , Morris says . War seems to be only way people have found to create peaceful societies , he argues . In the Stone Age , humans were a rough lot . Ian Morris : World has single great power -- USA -- capable of deterring governments from using force .
Since 10,000 years ago , Stone Age says the risk of violent death has fallen to 10 to 20 % . In perhaps greatest paradox in history , what 's made the world safer is war itself , Morris says . War seems to be only way people have found to create peaceful societies , he argues . Ian Morris : World has single great power -- USA -- capable of deterring governments from using force .