A few years ago I had a conversation at Harvard with Steven Pinker, the bestselling evolutionary psychologist. We were both writing optimistic books at the time, his being The Better Angels of Our Nature, about the decline in violence over recent centuries. I asked him: if all sorts of violence and cruelty were considered acceptable a century or two ago and are now beyond the pale — slavery, child labour, bear-baiting, wife-beating and so forth — then what routine habits do we practise today that we will look back on with horror in two or three generations’ time?

That’s easy, he replied: meat-eating. Don’t get me wrong, he added, I like meat, but the trend of history is clear, that one day in the future