What they can't agree on is whether the evolution of moral behaviour happened at the level of the group or the individual. There is, as Haidt admits, violent disagreement about this thesis among evolutionists. We approve of what is good for the group – our group. That's why our moral instincts are what Haidt calls 'groupish'. These were the people who belonged to groups in which individuals looked out for each other, rewarded co-operation and punished shirkers and outsiders. It comes to us from our ancestors, ie from the people whose instinctive behaviour gave them a better chance to survive and reproduce. Morality is not something we learn from our parents or at school, and it's certainly not something we work out for ourselves. Where do these moral instincts come from? Haidt is an evolutionary psychologist, so the account he gives is essentially Darwinian. They do not make more sensitive moralists.

What they are better at is coming up with what he calls 'side-arguments' for their own instinctive position. Haidt shows that people with high IQs are no better than anyone else at understanding the other side in a moral dispute. This is true no matter how intelligent we are. In fact, the main job of the rider is to come up with post-hoc justifications for where the elephant winds up. The rider can sometimes nudge the elephant one way or the other, but no one should be in any doubt that the elephant is making the important moves. The arresting image Haidt gives for our sense of morality is that it's like a rational rider on top of an intuitive elephant. What's more, in most instances, it's the outrage that will be setting the agenda. How dare anyone pass off such gratuitously offensive images as works of art? Shouldn't they be stopped? Jonathan Haidt, who gives a version of this thought experiment in his provocative new book, wants us to know that reason and instinctive outrage are always going to co-exist in cases like this. On the other hand, it would be surprising if they did not also feel disgusted and affronted. Liberals know the reasoned arguments for freedom of expression and the importance of being consistent on matters of principle. How would you feel if you walked into an art gallery and saw an image of King submerged in urine or Mandela smeared with excrement? Are you deeply offended by works of art such as Andres Serrano's Piss Christ, which depicts Jesus as seen through a jar of urine, or Chris Ofili's The Holy Virgin Mary, which shows Mary smeared with elephant dung? So offended that you think they ought to be banned and the galleries that display them prosecuted? No? OK, then try replacing the religious figures in these pictures with the sacred icons of progressive politics, people such as Martin Luther King and Nelson Mandela.

The righteous mind