Most people like to eat meat. As they grow richer they eat more of it. For individuals, that is good. Meat is nutritious. In particular, it packs much more protein per kilogram than plants do. But animals have to eat plants to put on weight – so much so that feeding livestock accounts for about a third of harvested grain.

Farm animals consume 8 per cent of the world's water supply, too. And they produce around 15 per cent of unnatural greenhouse-gas emissions. More farm animals, then, could mean more environmental trouble.