Yang's documentation of the facts is remorseless. The more leftist the provincial party boss, he establishes in pages of exhaustive tables, the greater the number of deaths. The western province of Sichuan, so productive it is called "Heaven's Pantry", suffered ten million dead from starvation, chiefly because its party chief was an unquestioning ultra, anxious to ingratiate himself with Mao. The starving millions ate rice straw, corn stalks, rats, sparrows, roots, tree bark, algae, egret droppings, insects, and in the last stages of desperation, clay or human flesh (30 cents per half kilogram).

The cynicism and brutal cunning of the Chairman emerge clearly. "Why aren't we being tough?" he said as the deaths piled up. "It's just a matter of having a little less pork for a time, or fewer hairpins and a shortage of soap." At the Lushan conference in 1959, when the disastrous effects of his dogma were already clear, delegates expected a retreat to be called. But Mao had things fixed. As in the Hundred Flowers period of 1956, he allowed the doubters to speak, then turned the meeting into an "anti-rightist" campaign, rather than one aimed at curbing the lunatic excesses of the Left.

The retreat came in 1962, when peasants regained garden plots and most communal kitchens closed. But that too was to prove tactical. The Cultural Revolution was a continuation of the Great Leap by other means, and it was then that Mao exacted revenge on his most courageous opponent at Lushan, defence minister Marshal Peng Dehuai.

Yang estimates the victims of the Great Leap at over 36 million, though he acknowledges that other sources, including an internal post-Mao report by the party itself, have come up with higher figures. As well as his scrupulous calculations about the numbers who died, Yang discusses the losses to the Chinese population as a whole through abortions or failure to conceive. In one district 60 per cent of women ceased menstruating, and 30 per cent suffered uterine collapse.

The scale of the ideologically inspired slaughter — some two-thirds of the population of Britain — is driven home. His figure for the dead, Yang points out, is similar to the number killed in the First World War, and almost as many as in the Second (40-50 million). Why has it taken so long for the enormity of Mao's carnage, and of his guilt, to emerge? The efficiency of Chinese censorship and repression is one reason. Another is the lack of photographic evidence, of the kind we have seen of Auschwitz. If there are no pictures, the modern mind reasons, can we be sure it ever happened?