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.

Photo: Getty
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Zac Goldsmith has bitten off more than he can chew

In standing as an independent, Goldsmith may face the worst of both worlds. 

After just 48 years, we can announce the very late arrival of the third runway at Heathrow. Assuming, that is, that it makes its way past the legal challenge from five local councils and Greenpeace, the consultation with local residents, and the financial worries of the big airlines. And that's not counting the political struggles...

While the Times leads with the logistical headaches - "Heathrow runway may be built over motorway" is their splash, the political hurdles dominate most of this morning’s papers

"Tory rebels let fly on Heathrow" says the i's frontpage, while the FT goes for "Prominent Tories lead challenge to May on Heathrow expansion". Although Justine Greening, a May loyalist to her fingertips, has limited herself to a critical blogpost, Boris Johnson has said the project is "undeliverable" and will lead to London becoming "a city of planes". 

But May’s real headache is Zac Goldsmith, who has quit, triggering a by-election in his seat of Richmond Park, in which he will stand as an anti-Heathrow candidate.  "Heathrow forces May into Brexit by-election" is the Telegraph's splash. 

CCHQ has decided to duck out of the contest entirely, leaving Goldsmith running as the Conservative candidate in all but name, against the Liberal Democrat Sarah Olney. 

What are Goldsmith's chances? To win the seat, the Liberal Democrats would need a 19.3 per cent swing from the Conservatives - and in Witney, they got exactly that.

They will also find it easier to squeeze the third-placed Labour vote than they did in Witney, where they started the race in fourth place. They will find that task all the easier if the calls for Labour to stand aside are heeded by the party leadership. In any case, that Clive Lewis, Lisa Nandy and Jonathan Reynolds have all declared that they should will be a boost for Olney even if she does face a Labour candidate.  

The Liberal Democrats are fond of leaflets warning that their rivals “cannot win here” and thanks to Witney they have one ready made.  

Goldsmith risks having the worst of all worlds. I'm waiting to hear whether or not the Conservatives will make their resources freely available to Goldsmith, but it is hard to see how, without taking an axe to data protection laws, he can make use of Conservative VoterID or information gathered in his doomed mayoral campaign. 

But in any case, the Liberal Democrats will still be able to paint him as the Brexit candidate and the preferred choice of the pro-Heathrow Prime Minister, as he is. I think Goldsmith will find he has bitten more than he can chew this time.

This article originally appeared in today's Morning Call, your essential email covering everything you need to know about British politics and today's news. You can subscribe for free here.

Stephen Bush is special correspondent at the New Statesman. His daily briefing, Morning Call, provides a quick and essential guide to British politics.