As you are reading this newspaper and not another, there is a good chance you may have wondered why some people you know, whose moral compasses seem otherwise to be functioning well, nevertheless vote for the Conservatives or their equivalent whenever offered the chance. This is the question Jonathan Haidt has set out to answer  and his conclusions may make unsettling reading for those of a liberal (American sense) persuasion.

Professor Haidt's premise is, as far as I can see, fairly easy to summarise: the reason republicans and conservatives persist in winning elections (if you discount Obama's last two victories, which I must say rather gum up the works of his argument) is because they appeal to a greater range of moral impulses than do more leftwing parties. Haidt claims that just as we have the taste receptors of salt, sweet, bitter, and so on, so we generally work on five basic moral receptors: those pertaining to caring, fairness, loyalty, authority and sanctity. (These terms vary: "purity" replaces "sanctity" on a website co-founded by Haidt, yourmorals.org, which, after a simple test, allows you to see how you scored in comparison with liberals or conservatives. It turns out that I am not as caring as I thought I was. Have a go  it's the foundation for the research that has gone into this book.)

Liberals are very big on caring and fairness, but tend not to mind so much about "sanctity". Conservatives, however, care about all these things. The more rightwing they are, the less bothered they are about fairness, and the more bothered they are about "sanctity". So for liberals to appeal more to everyone, and to win more elections, what they should do is press the buttons pertaining to good order and individual responsibility towards the herd harder than they do.

At which point I found that if I cupped my hand to my ear, I could hear the faint lowing of bullshit-providers in an adjacent field. I know Haidt has spent some years thinking about this, while all I have done is read his book, but I am not entirely convinced, and it is not just because the thought that Blue Labour or the Red Tories were on the right track is a dispiriting one.

The analogy he uses is of a mahout on an elephant: the rider can nudge the elephant in a certain direction, but if the elephant insists on going one way or another, there's not much that can be done about it. The rider, says Haidt, is our conscious intellect, and the elephant is our gut instinct, trained by brute evolution. Hence our ability to come up with any number of arguments that support our decision to believe in, say, lower taxes, or, conversely, a national health service. If you are a liberal baffled about why anyone would be offended by Andres Serrano's Piss Christ, imagine how you would feel if it were an image of Nelson Mandela suspended in a jar of the artist's urine.

Haidt has a point. We cannot be told often enough that we tend to shape or select the evidence in order to justify our convictions. To see the other person's point of view is not a bad thing. (And does anyone ever really mean it when they say: "When the facts change, I change my mind"?)

The problem is that this, of course, applies to Haidt, too. He has changed his mind  he used to be more dismissive of the right, and conservative cultures  but he is still in the same bind as the rest of us, choosing the evidence to support his beliefs. He can also be simplistic  personally speaking, I do not like Kant being flagged on the Asperger's spectrum because he went for a walk at exactly the same time every day. ("Out of the crooked timber of humanity no straight thing was ever made," he said. That, I buy.)

So I am in the odd position of being wary of a book I am also recommending. It's entertaining, snappily written and thought-provoking. It might even help Labour win the next election. But it still doesn't explain the gang running the country at the moment.

When Barack Obama won the Democratic nomination in the 2008 US presidential race, Jonathan Haidt was thrilled. After the inward-looking candidates chosen in previous races, here was a man able to speak to the centre and slaughter some sacred cows on his way to the White House. But as time went on, Haidt began to worry that once again his party's candidate was talking only to his own supporters.

So the social psychologist wrote an essay on why people vote Republican  and from that has evolved The Righteous Mind, which has been causing a stir in both Washington and Westminster.

Haidt looked at the usual ways psychologists explained away conservatism, such as strict parents or an overbearing fear of change. And he came to a radical conclusion: conservatives, rather than being victims of bad childhoods or possessing ugly personality traits, were just as sincere as liberals in wanting the best for society.

This may not sound such a startling statement. But many on the left are endlessly baffled as to why working-class voters seem to go against their own interests by supporting conservative politicians, those hated promoters of big business and tax cuts for the rich. They presume such voters are either stupid or are being tricked.

But the left's real problem, according to Haidt, is that it does not understand the motivations of the right. Drawing on everything from advertising to anthropology, he argues that liberals are driven by a morality based on compassion, the desire to fight oppression and, to some degree, fairness. Conservatives have a broader set of six "moral tastes", sharing such concerns but balanced by the binding foundations of loyalty, authority and sanctity.

It is, he says, as if the left has three taste buds but the right has six. While the right can "taste" issues such as compassion and fairness, the left struggles to embrace patriotism or religion, seeing traditional institutions and hierarchies as obstacles to their fight for liberty and equality. Haidt calls this "the conservative advantage".

Indeed, he goes further, saying that western progressives seeking a secular, rational society are out of step with the vast majority of people on the planet. He shows how our liberal values are "Weird"  supported only by those who are western, educated, industrialised, rich and democratic. He draws on visual perception studies to show how Weird and non-Weird people think differently and see the world differently, with those in the west putting far greater emphasis on individualism.

At heart, however, The Righteous Mind is an old-fashioned liberal plea for tolerance. Haidt readily admits that he set out to use moral psychology to help political partisans understand and respect each other. It is a welcome attempt to combat polarisation at a time when politics is descending into dysfunctional tribalism, a process speeded up by technology and changing residential patterns. It takes only one glance at the grocery store to determine a US town's politics: if you see a Whole Foods Market, for example, nine times out of 10 you are in a county voting Democrat.

What makes the book so compelling is the fluid combination of erudition and entertainment, and the author's obvious pleasure in challenging conventional wisdom. One minute he draws on psychological experiments to defend Glaucon, the cynic in Plato's Republic who argued that people behaved well only because they were scared of being caught. (Here Haidt gives dishonourable mention to Britain's MPs, so happy to abuse expenses when they thought no one was looking at their moats and duck ponds.) The next he is enlisting the Scottish philosopher David Hume to challenge our "rationalist delusion". He asks a series of strange questions  is it wrong to eat your dog if you run it over by accident, or to perform sexual intercourse on a dead chicken?  to prove how people rely on intuition to find answers, then produce reasons to justify them. Transcripts show how people tie themselves in knots arguing against incest, however much their arguments are torn apart. Reason, he concludes, is like a government press secretary, there to defend your decisions to others. "Anyone who values truth should stop worshipping reason."

Although Haidt glosses over the uncomfortable conclusions of what he is saying on issues such as race and human rights, his core point is simple and well-made: our morality, much of it wired into brains from birth, at the same time binds us together and blinds us to different configurations of morality. Gut feelings drive strategic reasoning, which can make it difficult to connect with those across the gulf, especially for liberals.

At the same time we are both selfish and groupish  as he puts it, nine parts chimpanzee for every one part bee. But we are not fundamentally selfish creatures, despite what we have been told for the past half century. We just need to encourage the bee to take flight, working a little harder at establishing trust and commonality with each other. "We're all stuck here for a while," he concludes. "So let's try to work it out."

Here's a thought experiment. 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. 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?

Many people are likely to feel torn. Liberals know the reasoned arguments for freedom of expression and the importance of being consistent on matters of principle. On the other hand, it would be surprising if they did not also feel disgusted and affronted. 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. What's more, in most instances, it's the outrage that will be setting the agenda.

The arresting image Haidt gives for our sense of morality is that it's like a rational rider on top of an intuitive elephant. 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. In fact, the main job of the rider is to come up with post-hoc justifications for where the elephant winds up. We rationalise what our gut tells us. This is true no matter how intelligent we are. Haidt shows that people with high IQs are no better than anyone else at understanding the other side in a moral dispute. What they are better at is coming up with what he calls "side-arguments" for their own instinctive position. Intelligent people make good lawyers. They do not make more sensitive moralists.

Where do these moral instincts come from? Haidt is an evolutionary psychologist, so the account he gives is essentially Darwinian. Morality is not something we learn from our parents or at school, and it's certainly not something we work out for ourselves. We inherit it. It comes to us from our ancestors, ie from the people whose instinctive behaviour gave them a better chance to survive and reproduce. These were the people who belonged to groups in which individuals looked out for each other, rewarded co-operation and punished shirkers and outsiders. That's why our moral instincts are what Haidt calls "groupish". We approve of what is good for the group  our group.

There is, as Haidt admits, violent disagreement about this thesis among evolutionists. What they can't agree on is whether the evolution of moral behaviour happened at the level of the group or the individual. At lot hangs on the answer, including whether altruism is at root selfish (it gives individuals and their genes the best chance of surviving) or not (it involves genuine individual sacrifices for others). But not a lot hangs on it for Haidt's argument, since his concern is simply to establish that this is the way we have turned out, not how we got here. He could add that the animosity between the two sides in this academic dispute  some of the protagonists really seem to have contempt for each other  shows how even the most intelligent riders can be under the control of their elephants.

Haidt wants us to understand that our moral instincts are inherently judgmental: being moral makes us moralistic. Much of the book is devoted to the experimental evidence that shows how often moral judgment is a case of us v them rather than right v wrong. In Haidt's terms, morality "binds and blinds". It binds us to the group and blinds us to the point of view of outsiders. This has profound implications for how we might think about some of our most deeply held beliefs. For instance, it means that what we believe is less important than with whom we share those beliefs. Haidt thinks this is particularly true of religion and it is why he thinks the arguments of the current crop of militant atheists (Dawkins, Hitchens et al) are misguided. They spend their time fretting about the irrationality of religious belief and ignore the fact that religion is about shared values and a sense of solidarity. Religion, Haidt says, is a "team sport". In one of the many striking images in this book, he suggests that "trying to understand the persistence and passion of religion by studying beliefs about God is like trying to understand the persistence and passion of football by studying the movement of the ball."

He also thinks this understanding of morality has important things to tell us about politics. This is where his argument is less persuasive. One of Haidt's claims is that the moral appeal of the left in contemporary politics is too narrowly confined to issues of justice and equality, whereas the right can speak the language of loyalty and authority. He thinks left-leaning politicians should expand their groupish repertoire. What he doesn't say is how they can do this without seeming to be pandering to the other side. The recent, cack-handed attempt by "blue Labour" thinkers? to come up with a version of progressive politics that speaks to our instinctive loyalties and our suspicion of freeloaders shows how hard it is to get it right. Haidt is not wrong in principle but he makes it sound too easy.

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.

This book has quite a lot in common with another recent work of popular political psychology, Drew Westen's hugely influential The Political Brain. Westen argued that people on the left in politics spent too much time trying to reason with the voters, oblivious both to how little impact this had and to how righteous it made them sound. Westen insisted that their arguments had to be framed as emotionally engaging stories if people were going to hear what they wanted to say. Haidt has a richer and more interesting explanation for the same phenomenon: his is in many ways the more interesting book. But the story he tells is a scientific one. It's not going to cut it as politics.

 David Runciman's Political Hypocrisy is published by Princeton.