How are we supposed to believe manifesto costing promises when they aren't independently assessed?

The assertion that Labour would bring in £6.4bn by clamping down on tax avoidance is simply a made-up number, in the sense that it’s an aspiration, rather than being based on any kind of programme that can be evaluated

Jeremy Corbyn unveiled the Labour Party manifesto (Nigel Roddis/EPA)

“Annual income twenty pounds, annual expenditure nineteen pounds nineteen and six, result happiness. Annual income twenty pounds, annual expenditure twenty pounds nought and six, result misery.”

General elections tends to turn journalists and broadcasters into Dickens’s Mr Micawber, albeit without the charm. Do the promises of the parties add up? Is there a gap? Is there to be happiness or misery? The questions ring in our ears.

In many ways it’s a silly and unedifying spectacle. As the economist Chris Dillow has pointed out, it’s a sham to imply that such “costings” exercises can tell us anything about the how the public finances will evolve under any particular government.

That depends far more on the state of the economy. And to the extent that the costings obsession of journalists at election time distract attention from bigger questions of macroeconomic management, it’s harmful.

If growth is crushed because the government imposes excessive austerity while interest rates are still at rock bottom – something close to what we saw in 2010 under the Coalition – even the most honest of manifestos and most accurate of costings are not going to help the deficit.

In 2010 the Conservative manifesto pledged to eliminate “the bulk" of the current structural budget deficit by 2015. In fact it was still £45bn in that year, mainly because the economy performed so badly.

Yet at the same time, political parties should not be allowed to promise higher public spending or redistribution without acknowledging the costs and trade-offs. If that sounds like an anti-progressive conspiracy, consider how right-wingers are prone to making assertions about how cutting taxes will magically pay for themselves by turbo-charging growth. The fiscal credibility question cuts both ways. Or at least it ought to.

Labour’s tax costings today are a mixed bag. The income tax (£6.4bn) and corporation tax (£19.4bn) raising figures by 2021-22 look broadly reasonable because they are based on a HMRC “ready reckoner” document, which allows anyone to estimate what changing headline tax rates would mean for revenues.

But the assertion that Labour would bring in £6.4bn by clamping down on tax avoidance is simply a made-up number, in the sense that it’s an aspiration, rather than being based on any kind of programme that can be evaluated. We saw precisely the same made-up numbers in the 2015 manifestos from both Labour and the Conservatives.

Falling between those two extremes are Labour’s estimates that an “excessive pay levy” would raise £1.3bn or that introducing a new financial transactions tax would bring in £5.6bn, to take just two examples. This is speculative because we cannot say with any confidence how the public’s behaviour would change in response to the introduction of such new taxes because we have no history to go on.

Would firms simply soak up the new pay levy in the form of lower profits and carry on rewarding top staff in the same way? Or would they curb salaries, meaning the levy raised negligible amounts for the taxpayer?

The same applies to the proposed transaction tax. Perhaps asset managers and financiers would trade less in response to the levy, meaning it doesn’t produce much money. Incidentally, given Labour regards both excessive pay and excessive financially trading as undesirable, it logically ought to welcome a strong behavioural response – although that would create a problem for its costings.

It’s true, of course, that new taxes are introduced by governments all the time. And governments, when they do this, always make an estimate of how much money it will end up raising, taking into account behavioural change. Yet there’s a check on over-optimism now in the shape of the Office for Budget Responsibility. The OBR tells the Treasury and HMRC to think again if it isn’t convinced by their estimates. And it highlights the uncertainty of particular costings.

The obvious and sensible solution to the issue of election manifesto costings is to allow the OBR to perform the exercise – applying the same uncertainty scale on individual tax proposals as it does at Budgets.

This isn’t a particularly radical suggestion. The OBR’s equivalent in the Netherlands already costs the manifestos of parties that submit their proposals to it in good time. And the head of the OBR, Robert Chote, has said his organisation is willing to do the job, provided its resources are significantly expanded.

The former Chancellor George Osborne deserves credit for establishing the OBR in 2010. The watchdog has helped restore credibility and transparency to Budgets. But Osborne turned down a proposal from Labour in 2014 to allow the OBR to cost all the party manifestos.

Whoever forms the next government would be wise to revisit this. The results for the voting public might not be Micawberite ecstasy, but we would certainly be better informed about the choices available to us than we are now.