HOW do politicians get away with bending the truth? The answer may lie with a fundamental psychological tool that we use to make sense of the world.

The current US presidential campaigns have featured untruths on both sides that have been notable for their durability. Politicians have been exposed for misrepresenting their position and then kept at it anyway, making the same misleading statements again and again.

Sarah Palin has taken much flak for this. To prove she is a fearless fiscal conservative, Palin touts her previous resistance to the "Bridge to Nowhere", a multimillion-dollar link between mainland Alaska and an island that is home to a small airfield and 50 inhabitants. In reality, Palin backed the bridge, even after Congress scrapped the project. Journalists have pointed this out, yet Palin continues to voice her apparent opposition.

Democrats are also at it. They've repeatedly said that McCain is in thrall to free-market thinking. He wants to privatise social security, warns Barack Obama. That will put the savings of millions of elderly people at the mercy of stock market fluctuations. "Balderdash," says the non-partisan FactCheck.org, based at the University of Pennsylvania; McCain is actually proposing moderate reforms that would not affect the current generation of senior citizens.

How do politicians get away with this? Ignorance is part of the answer. Many voters will never read the newspaper article or watch the news broadcasts that reveal the true situation. But psychology is also at work. The short cuts that we use to make sense of the world shape our perception of it. When it comes to politics, this can lead voters to reach the wrong conclusions about candidates, even when they have been exposed to the truth. Could it be that politicians and their strategists are harnessing this phenomenon?

The origin of this is a seemingly mundane psychological finding: we tend to arrange the world into categories. This saves thinking time. Even the least-engaged voter knows that McCain is a Republican, for example. When the voter places him in that category, their brain automatically links McCain to attributes shared by other Republicans. The voter might not recall McCain's position on the 2003 invasion of Iraq, for example, but they would assume that as a Republican he supported it - which he did.

Problems arise when we try to recall something that does not fit with the categories we use. Experiments conducted in the 1980s by Milton Lodge and Ruth Hamill at the State University of New York at Stony Brook examined how beliefs and stereotypes, such as those associated with gender or race, affect the way that voters analyse candidates. They found that correct information about a candidate was often forgotten or misinterpreted if it conflicted with the way voters categorised that politician.

It is this that the campaigns are tapping into when they release false information. Palin's misstatements on the Bridge to Nowhere have not attracted much attention with voters in spite of stoking media discussion, because Palin is a Republican and so is expected to want to cut back on government spending. For the same reason, Obama's inaccurate statements about McCain's social spending plans do not sound wrong, even though they are.

The categorising process, which has been shown to help explain how we learn and remember things, has now been modelled for political beliefs by Nathan Collins, a political scientist at the Santa Fe Institute in New Mexico. In a paper being considered for publication by The Journal of Politics, he finds that voters are more likely to misremember a candidate's position if it conflicts with the party line. And that, says Collins, opens the door to deceptive campaigning. FactCheck.org, it seems, should be compulsory reading for any American with a vote.