A year ago The Spirit Level made a persuasive case for radical income redistribution. Since then it has been enthusiastically adopted as the new bible of progressive social democracy. A look at some of the language used by the Labour leadership candidates reveals that Ed Miliband, in particular, has been heavily influenced by the book, to the extent that he has constructed the core of his campaign around the issue of equality. Last week, however, saw the beginning of the backlash: centre-right think tank Policy Exchange published a 125-page critique of The Spirit Level called, rather ominously, Beware False Prophets. The pamphlet was written by Peter Saunders, professor emeritus of Sociology at the University of Sussex. Saunders’ pamphlet has already been vilified on the Guardian website by The Spirit Level’s authors as a ‘hatchet job’, and ‘racist’. The Spirit Level, then, has garnered strong reactions from both sides of the political divide – but what explains the remarkable impact it has had on political discourse?

Few issues are quite so divisive as the question of inequality; indeed, where you stand on this particular battleground is perhaps the ultimate litmus test of one’s political persuasion. The case for equality has been fought and re-fought for a very long time now, and in this respect there is nothing new about The Spirit Level. The book, however, does bring a rather novel weapon to the left’s arsenal – namely, the claim that inequality damages the rich as well as the poor – and then supports this with an abundance of empirical evidence drawn from impartial and reliable UN statistics.

The Spirit Level’s central thesis is that, as the gap between rich and poor widens, problems affecting the whole of society worsen. The book’s authors, Richard Wilkinson and Kate Pickett, produce graphs which seem to show scientifically that less egalitarian societies are afflicted by higher rates of crime, infant mortality, obesity, and lower life expectancy (to name but a part of the catalogue of social ills blamed on inequality in the book). Wilkinson and Pickett argue that, because of this, it would be in everyone’s interest to strive to create a society in which wealth was distributed more evenly.

No wonder, then, that The Spirit Level has enjoyed so much support and exposure: here is a book which provides a scientific basis for the policies that the left traditionally advocate, seemingly denying any counter argument through sheer weight of evidence. What is more, in a move that further reduces the possibility of opposition to their ideas, Wilkinson and Pickett argue that no one would lose out as a result of redistributive taxation: we would all be happier, healthier and safer.

The position of The Spirit Level, then, seems almost unassailable. Ed Miliband has certainly been won over: on July 12th’s Today Programme he said ‘what we’ve got to realise is that all of us, not just the poor, suffer from the gap that there is between rich and poor… the countries that are healthier, happier, more secure – they are the more equal countries…’. In a short interview in which he outlined his plans to replace the minimum wage with a ‘living wage’, and talked about the introduction of a ‘high-pay commission’ to reduce top salaries, he said that he would make increased equality ‘a central aim’ of his leadership.

Just four days before Ed Miliband made these comments, Beware False Prophets was published. In it, academic Peter Saunders questions Wilkinson and Pickett’s handling of data and the conclusions they come to. He writes: ‘the statistical analysis in The Spirit Level is heavily flawed. There are many instances where graphs are presented in which just one or two extreme cases are used to support unwarranted generalisations.’ Saunders’ paper provides a valuable dose of balance, and it should be noted that it does not attack the ideal of equality per se. He is not making a political argument – he simply casts a critical eye over Wilkinson and Pickett’s scientific claims. Sad, then, to see such a vituperative article on the Guardian website as their response.

In one sense it is strange that the debate about equality is now phrased in scientific terminology. Perhaps we should get used to this, though: I predict that the biggest consequence of The Spirit Level will be the growing use of empiricism as a political tool. It is only a matter of time before further books come out on the back of its success, mimicking its evidence-based approach to politics. Which leads us to ask – how much of this kind of political ‘science’ is actually rhetoric in disguise? It seems to me that supplementing what is essentially a political argument with a bombardment of statistics and leaden analysis risks missing the point: equality is, and always has been, an ethical issue.

It is a fairly grim state of affairs if the public are persuaded more by a supposedly scientific justification for equality than by an appeal to moral principles. The left will do themselves no favours by using dubious science as their weapon. They risk abandoning what should be their strongest armament of all – the moral high ground. Politicians like Ed Miliband should take great caution not to use The Spirit Level alone – persuasive though it undoubtedly is – as a blueprint for policy. They should distinguish between rhetoric and science, and should not be afraid to use the language of morality even though it may buck the current trend.