The problem is that having spent most of the book showing how hard it is to get us to think rationally about morality, Haidt then tries to get us to see reason about politics. This is an American book and it’s the current state of American politics that Haidt wants to remedy. He despairs of its extreme partisanship and the toxic levels of mistrust on both sides. But his analysis can neither explain nor cure this phenomenon. He can’t explain it because it is relatively recent – the partisanship has got much worse in the last couple of decades – so it is not something that can be accounted for by evolution. People are predisposed to be divided by morality, but if we have suddenly become more divided that can’t be explained by our predispositions. Something else must have happened: changes to the role of money, or technology, or communication, or party organisation, or voting habits. In other words, the explanation is political not evolutionary.

The way Haidt wants to cure it is to have people understand that the divisions have gone too far. He flags up some traditional leftish arguments that might make sense to people on the right, and some rightish arguments that could appeal to the left. But he sets them out in essentially evolutionary terms: for instance, he wants people on the right to recognise the need for tighter government control of corporations because corporations are “super-organisms”. How’s that going to go down in a focus group? His hints at practical reform are equally unconvincing. He says that it would be better if politicians came to Washington with their families so that they would be forced to socialise with the other side. But why does he think that the families would choose to socialise with the other side rather than with people like themselves? Everything he says in the book suggests that people cleave to their own when their moral judgments are on the line.