The most important paragraph in the IMF World Economic Outlook

68 words of wonkishness.

The IMF's World Economic Outlook (pdf) – a 230-page tome detailing predictions on nearly every aspect of the world's economy collated by the international organisation – always gets attention for the calls it makes.

The October edition downgrades expected global growth for 2013 from 3.9 per cent to 3.6 per cent, and also cuts predictions for China (down to 8.2 per cent for 2013), the US (expected to grow by 2.1 per cent in 2013, down from 2.3 per cent in July's prediction) and the UK (now expected to grow by just 1.1 per cent next year, and to contract by 0.2 per cent this year).

But the predictions are not the most important passages in this edition of the Outlook. Those are found in a short box-out titled Are We Underestimating Short-Term Fiscal Multipliers?

The fisc§al multiplier is the effect government spending has on GDP. Money spent by the government doesn't disappear – it is respent, again and again. If a teacher gets a pay rise, their consumption is likely to rise in line with it; if all teachers get pay rises, that increase in consumption may be enough to affect the aggregate demand in the economy. In an economy which isn't being stretched to its limits – that is, one without full employment, or serious capital equipment shortages – that increase in aggregate demand will result in an increase in GDP.

The existence of the fiscal multiplier is a matter of fact, but the magnitude of it is contested. And that's where the IMF enters the scene, on page 42:

The main finding, based on data for 28 economies, is that the multipliers used in generating growth forecasts have been systematically too low since the start of the Great Recession, by 0.4 to 1.2, depending on the forecast source and the specifics of the estimation approach. Informal evidence suggests that the multipliers implicitly used to generate these forecasts are about 0.5. So actual multipliers may be higher, in the range of 0.9 to 1.7.

Emphasis mine. When deciding how much to spend, governments have been assuming that every pound they spend increases GDP by 50p – but it may increase it by as much as £1.70.

The reason this is so very important is that fiscal multiplier is usually appealed to not when deciding how much to spend, but how much to not spend. When governments are planning austerity packages, they have to be wary of the fact that large cuts to government spending will inevitably cause a decrease in output, and so they either have to be prepared to take that hit, or come up with a reason why slashing spending will cause an increase in output through some other mechanism.

That is easy enough to do if you are trying to account for a fiscal multiplier of 0.5: you can make the arguments, which Osborne and Cameron rehearsed repeatedly, that the public sector is crowding out the private; that the government spending which is being cut is particularly inefficient; or that the confidence fairies will reward your thriftiness with growth.

When there is the chance that the fiscal multiplier is three times that, austerity becomes much more likely to involve damaging drops in output.

There was once a time when the government pegged its credibility to that of the IMF – back when George Osborne was proud about Britain's credit ratings, and the international community was behind his plans. Those days are gone, and have been since Christine Lagarde made her own attack on austerity. But the economists at the treasury may be more inclined to listen to the wonkish findings of the World Outlook than the political interventions of the fund's leader. We can only hope they are prompted to re-do the sums.

The IMF headquarters. Photograph: Getty Images

Alex Hern is a technology reporter for the Guardian. He was formerly staff writer at the New Statesman. You should follow Alex on Twitter.

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Jeremy Corbyn challenged by Labour MPs to sack Ken Livingstone from defence review

Former mayor of London criticised at PLP meeting over comments on 7 July bombings. 

After Jeremy Corbyn's decision to give Labour MPs a free vote over air strikes in Syria, tonight's Parliamentary Labour Party (PLP) meeting was less fractious than it could have been. But one grandee was still moved to declare that the "ferocity" of the attacks on the leader made it the most "uplifting" he had attended.

Margaret Beckett, the former foreign secretary, told the meeting: "We cannot unite the party if the leader's office is determined to divide us." Several MPs said afterwards that many of those who shared Corbyn's opposition to air strikes believed he had mishandled the process by appealing to MPs over the heads of the shadow cabinet and then to members. David Winnick declared that those who favoured military action faced a "shakedown" and deselection by Momentum activists. "It is completely unacceptable. They are a party within a party," he said of the Corbyn-aligned group. The "huge applause" for Hilary Benn, who favours intervention, far outweighed that for the leader, I'm told. 

There was also loud agreement when Jack Dromey condemned Ken Livingstone for blaming Tony Blair's invasion of Iraq for the 7 July 2005 bombings. Along with Angela Smith MP, Dromey demanded that Livingstone be sacked as the co-chair of Labour's defence review. Significantly, Benn said aftewards that he agreed with every word Dromey had said. Corbyn's office has previously said that it is up to the NEC, not the leader, whether the former London mayor holds the position. In reference to 7 July, an aide repeated Corbyn's statement that he preferred to "remember the brilliant words Ken used after 7/7". 

As on previous occasions, MPs complained that the leader failed to answer the questions that were put to him. A shadow minister told me that he "dodged" one on whether he believed the UK should end air strikes against Isis in Iraq. In reference to Syria, a Corbyn aide said afterwards that "There was significant support for the leader. There was a wide debate, with people speaking on both sides of the arguments." After David Cameron's decision to call a vote on air strikes for Wednesday, leaving only a day for debate, the number of Labour MPs backing intervention is likely to fall. One shadow minister told me that as few as 40-50 may back the government, though most expect the total to be closer to the original figure of 99. 

At the end of another remarkable day in Labour's history, a Corbyn aide concluded: "It was always going to be a bumpy ride when you have a leader who was elected by a large number outside parliament but whose support in the PLP is quite limited. There are a small number who find it hard to come to terms with that result."

George Eaton is political editor of the New Statesman.