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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PMQs review: Theresa May shows how her confidence has grown

After her Brexit speech, the PM declared of Jeremy Corbyn: "I've got a plan - he doesn't have a clue". 

The woman derided as “Theresa Maybe” believes she has neutralised that charge. Following her Brexit speech, Theresa May cut a far more confident figure at today's PMQs. Jeremy Corbyn inevitably devoted all six of his questions to Europe but failed to land a definitive blow.

He began by denouncing May for “sidelining parliament” at the very moment the UK was supposedly reclaiming sovereignty (though he yesterday praised her for guaranteeing MPs would get a vote). “It’s not so much the Iron Lady as the irony lady,” he quipped. But May, who has sometimes faltered against Corbyn, had a ready retort. The Labour leader, she noted, had denounced the government for planning to leave the single market while simultaneously seeking “access” to it. Yet “access”, she went on, was precisely what Corbyn had demanded (seemingly having confused it with full membership). "I've got a plan - he doesn't have a clue,” she declared.

When Corbyn recalled May’s economic warnings during the referendum (“Does she now disagree with herself?”), the PM was able to reply: “I said if we voted to leave the EU the sky would not fall in and look at what has happened to our economic situation since we voted to leave the EU”.

Corbyn’s subsequent question on whether May would pay for single market access was less wounding than it might have been because she has consistently refused to rule out budget contributions (though yesterday emphasised that the days of “vast” payments were over).

When the Labour leader ended by rightly hailing the contribution immigrants made to public services (“The real pressure on public services comes from a government that slashed billions”), May took full opportunity of the chance to have the last word, launching a full-frontal attack on his leadership and a defence of hers. “There is indeed a difference - when I look at the issue of Brexit or any other issues like the NHS or social care, I consider the issue, I set out my plan and I stick to it. It's called leadership, he should try it some time.”

For May, life will soon get harder. Once Article 50 is triggered, it is the EU 27, not the UK, that will take back control (the withdrawal agreement must be approved by at least 72 per cent of member states). With MPs now guaranteed a vote on the final outcome, parliament will also reassert itself. But for now, May can reflect with satisfaction on her strengthened position.

George Eaton is political editor of the New Statesman.