Chris Giles in the FT says that, underneath the bluster, "the parties have rarely been closer on economics" - which is just what Hotelling's law and the median voter theorem predict. This raises the question: why, then, do political partisans pretend otherwise?

Some of the reasons are innocuous. Salesmen rarely claim there's little difference between their products and their rivals', and there are a host of cognitive biases - such as the optimism bias and groupthink - which cause them to oversell their own policies. And we might add to this the narcissism of small differences.

But there's something more subtle going on. There's a selection bias here. Most people who become interested in politics do so because they think policies matter. As a result, discussion of politics is disproportionately dominated by people who are prone to exaggerate the impact of policy.

Such talk, however, overlooks some important points. There's Adam Smith's insight that there's "a great deal of ruin in a nation" - that we can cope tolerably well with suboptimal policies (which is just as well). There's the evidence that national policies don't much affect trend GDP growth. And then there are the myriad structural and ideological constraints that limit what any government can do.

But here's the problem. This selection bias isn't just confined to political partisans. It also applies - generally speaking; there are exceptions - to political journalists (Chris, remember, is an economics writer). The guy who thought party politics was unimportant wouldn't become a political journalist in the first place, and wouldn't last long if he did.

In this sense, even journalism which aspires to be "neutral" is in fact biased. It's biased towards exaggerating differences: "clash", "row" and "deep divisions" are standard journalese. It's biased towards exaggerating the importance of such "divisions": how often do you hear Nick Robinson say "this doesn't really matter"? And it's biased towards giving the oxygen of publicity to those who think party politics matters, and thus underweights sceptical voices like Chris's.

These biases might be pernicious, insofar as they help to distract us from important facts - such as the possibility that, within capitalism, there's not often very much that any political party can (consciously?) do to transform the economy or society.