At the heart of this work is a startling and improbable statistic and the equally surprising and counterintuitive thesis that flows out of it. We are used to looking back on the 20th century as comfortably the most violent in all human history — the silver medal usually goes to the 14th — but if Ian Morris(a fellow at Stanford University) is to be believed, the century that could wipe out perhaps 50 million to 100 million in two world wars and throw in the gulags, the Cultural Revolution, civil wars, government-orchestrated famine, trench-stewed pandemics and any number of genocides...