Yet killings in conflict, the curse of humanity since the ancient world, have been in marked decline in the past two decades. Professor Pinker calls this development the “New Peace”. He is fond of graphs and the accumulated data is compelling. Deaths due to state-based conflicts show jagged peaks in the two world wars, followed by a bumpy but consistent trailing-off. The 20th century, with its trio of premier-league killers, Hitler, Stalin and Mao, stands out for the sheer scale of the destruction of human life. Technology, in the form of Zyklon B and cheap transport, enabled dictators to dispatch their victims en masse to distant killing fields. But these mass killings ran counter to a general trend of pacification.