Should the BBC allow politicians onto its programmes? I'm prompted to ask by Michael Portillo's appearance on Broadcasting House yesterday. He claimed that the fact that 1% of taxpayers pay 28% of income tax is evidence that the tax system is progressive. This is just plain wrong - not as a matter of opinion, but of fact and logic. Yes there's other evidence that income tax is mildly progressive, but the share of tax paid by the top 1% is irrelevant.

But nobody challenged Portillo on this error.

Which is a reason why politicians should not be given airtime. Doing so carries a high risk that they will just spout idiotic untruths; see also Chris Grayling's weird invocation of the SWP or Nadine Dorries, passim. (I'm just giving examples that spring immediately to mind - I don't doubt that Labour politicians are also guilty*.)

It's not good enough here for the BBC to reply that it usually aims at 'balance' by having opposing politicians on. This can lead to what Paul Krugman called 'opinions differ on shape of planet' journalism. If one party says the world is flat, and another says it isn't, you do not have a reasoned debate, but just one moron and one non-moron. In this sense, there's a conflict between the BBC's mission to inform and educate on the one hand, and it's aim at impartiality on the other; being impartial between truth and falsehood does not inform or educate.

Take some of the big political issues: could workfare work? Is there a case for reforming the NHS? Is fiscal austerity justified? Any of us could think of a few hypotheses or anecdotes to support either side of these arguments. Having politicians do the same does not add value. What we want to know is the evidence - and the holes in that evidence. This requires that the BBC have experts on their shows, not politicians**.

Nor am I sure that having politicians appear even does the politicians any favours. On the one hand, the mere exposure effect suggests that their regular appearances garners support. On the other hand, though, because they are usually overconfident about they abilities, they over-estimate the extent to which they can make a favourable impression. In this case, banning politicians from the BBC would protect them from themselves.

One objection to this is that politicians matter not because they inform or educate, but because they represent popular opinion. I'm not convinced. The fact that an opinion is mainstream - and it's questionable whether politicians do represent the mainstream - is a reason for not broadcasting it. We know what we think; what's the point of hearing an echo?

Now, I'm not saying there should be a blanket ban. Where there is a genuine debate about values - for example security vs. freedom or equality vs. incentives - it's reasonable to have politicians' debate them. But when the debate is about issues of social science, the BBC should not waste scarce airtime on them. And many political issues are ones of science rather than values.

* I never watch current affairs programmes on TV - hence my lack of other examples.

** I'm not saying it should have me on. I usually turn down the few invitations I get to appear.