The coalition defined itself as a government of austerity or, as its members preferred, as a government with the courage to take the hard decisions necessary to deal with the deficit. In its first two years it did what it had promised to do – and more – and as a result inflicted palpable harm on the economy. The recovery was delayed, costing the average household the equivalent of at least £4,000. In 2012 the government departed from its earlier plans and eased up on austerity, but pretended it had not.

The numbers are stark. GDP per head, a far better indicator of prosperity than GDP alone, grew on average by just 1 per cent a year between 2010 and 2014. The average growth rate from 1950 to 2010 was close to 2.25 per cent. Even under the last Labour government, average growth was 1.5 per cent, and that period included the global financial crisis. The past few years, as we recovered from the crash, should have been a time of above-average, not below-average growth. Even growth in the past two years has been only average by historical standards.

A government entering an election with that kind of performance should be trying to avoid talking about its economic record at all costs. Yet the opposite is the case. Indeed, the Conservative Party has an election platform that promises to repeat exactly the same mistake it made 2010. As a macroeconomist, I find it very easy to explain the impact the government’s mistakes had on the economy. I find it much more difficult to understand how it might, in three weeks’ time, get away with them, let alone promise to make the same mistake again.

The first important point to note is that austerity was not forced on the coalition. There was no market pressure that required it to embark on rapid fiscal tightening. There was a government debt crisis in 2010 but it was confined to a few eurozone countries, for one simple reason: none of those countries has a central bank of its own. If the markets refused to fund their governments they could not ask their own central bank to do so instead. From 2010 until September 2012, the European Central Bank refused to play the role that economists call “lender of last resort” and as a result interest rates on Irish, Portuguese and Spanish government debt increased substantially. In September 2012, the ECB changed its mind and promised (with conditions) to act as a lender of last resort. Interest rates fell and the eurozone debt funding crisis came to an end.

Outside the eurozone, governments had no problem funding their deficits. Interest rates on UK debt and that of other countries fell steadily. Yet to listen to many City economists is to be told that we should not take the markets for granted. Had austerity not been imposed, these markets could have turned on us at any time, and therefore it was right to reduce the deficit sharply as a precautionary measure. There is, unfortunately, a good deal of self-interest in this advice. If we have to fashion our economic policy to appease an unpredictable market, it adds to the influence of those who profess to be able to interpret its mood.

So let us imagine what might have happened, had the UK not undertaken austerity in 2010 and if the markets had started to worry that it might default. That would have put upward pressure on interest rates, as markets required some compensation for the possibility of default. However, the Bank of England was at the same time buying large quantities of UK government debt under its quantitative easing (QE) programme, which was designed to keep rates low. Any market panic would have been quickly offset by the Bank’s actions as it bought more debt. Unlike eurozone countries, the UK can never “run out of money” and so is not at risk of default.

Embarking on austerity was a choice for the coalition, not something it was forced to do. But large deficits cannot be sustained permanently. At some point they need to be reduced. And yet, since the time of Keynes, standard economics has recognised that cutting government spending or raising taxes reduces aggregate demand. So is there ever a good time to reduce the deficit?

There is a simple answer to that question. Although cutting the deficit will reduce demand, this can be offset by the central bank cutting interest rates. Fiscal austerity need not damage the aggregate economy as long as monetary policy is able to push in the other direction. The big problem in 2010 was that this was impossible because interest rates were already as low as the Bank thought prudent. So there is one set of circumstances in which it is unwise to cut the deficit and these circumstances were exactly those that prevailed in 2010.

Although the Bank felt it could not cut interest rates any further, it did have the policy of QE. Could this substitute for the inability to cut short-term interest rates? The answer is that economists had very little idea, essentially because QE had not been tried before. To embark on austerity, and hope that the programme would offset its effects, was therefore a large risk to take.

What happened was that the recovery in output that seemed to be about to occur in 2010 did not materialise. George Osborne would say that this poor performance was the result of things outside his control, such as the eurozone crisis. However, here we can turn to the Office for Budget Responsibility for guidance. The OBR calculates that austerity reduced GDP growth by 1 percentage point in both of the first two years of the coalition government: therefore, the level of GDP was 2 points lower in the second year. As growth did not return until 2013, at the very least that indicates that austerity led to a cumulative output loss of 5 per cent of GDP, which is about £4,000 per household.

How firmly based is the OBR analysis? There are very good reasons for thinking that its numbers are rather conservative. They look at the average effect of austerity over the past but, as has been noted, monetary policy is often able to offset the impact of fiscal consolidation on output, whereas on this occasion monetary policy’s hands were tied. We also have good econometric evidence that austerity has a larger-than-average impact in periods of recession. So, you could easily double the £4,000 number.

Osborne originally intended to eliminate the deficit within five years. However, in 2012, with the recovery nowhere in sight and tax revenues lower than expected, he changed the plan. Since 2012 there has been much less deficit reduction and, partly as a result, the recovery began – three years late – in 2013.

***

This is all straightforward economics of the kind taught to every economics undergraduate around the world. The government chose a policy that many economists said in advance would do considerable harm. When that harm materialised it had to change its policy. That should have meant the government suffered a large blow to its reputation. The delayed recovery is one reason why living standards have suffered, so this is hardly an academic issue. A government with this woeful record should not be campaigning on economic competence. So, how has it managed to turn complete failure into the appearance of success?

There are four critical steps in how this was achieved. The first was to equate government budgets with household budgets. A consequence of recession is that many individuals and firms have to tighten their belts, so it seems intuitive that governments should do the same. This will be painful but individuals know that putting off their own adjustment can make things worse. It is part of every economics student’s initial education to learn why this analogy between individuals and governments is wrong – but most people have not studied economics.

A second key step was to blame the deficit on Labour profligacy. You do not need an economist to tell you that the main reason for the increase in the deficit was the recession created by the financial crisis. It is the case that the later years of the Brown chancellorship were not as fiscally prudent as his earlier years. But just before the recession the government debt-to-GDP ratio was lower than in 1997, which hardly indicates profligacy. Some have tried to suggest in hindsight that 2007 was a massive boom year (implying the need to run a budget surplus) but most evidence suggests otherwise and that certainly was not what most people thought at the time. There is enough here to make the profligacy charge vaguely credible, however, to people who do not look at the numbers.

The third stage in the austerity deception was to pretend that the policy change in 2012 was not a change in policy. The truth is plain to see in the data, but it was vital for Osborne not to admit that he was easing up on austerity. If he had admitted to changing his policy, he would have had to say why: austerity was delaying the recovery. All this stuff about a “long-term economic plan” can be seen as part of the effort to cover up the reversal and, therefore, the austerity mistake.

Pretending there had been no change in policy also allowed the fourth and final stage of turning failure into success, which was the most audacious deception of all. This was to claim that the recovery in 2013 vindicated the austerity policy. To see how absurd this claim is, imagine that a government on a whim decided to close down half the economy for a year. That would be a crazy thing to do, and with only half as much produced, everyone would be much poorer. However, a year later when that half of the economy started up again, economic growth would be around 100 per cent. The government could claim that this miraculous recovery vindicated its decision to close half the economy down the previous year. That would be absurd, but it is a pretty good analogy to claiming that the recovery of 2013 vindicated the austerity of 2010.

This was how the government could turn economic failure into apparent political success. The strategy also had one further consequence. It redefined the meaning of what good macroeconomic policy was. If you asked any economist what the aim of government policy should be, he or she would probably say it was to increase the welfare of the public, or, more specifically, to raise standards of living. A government that had presided over the longest fall in real wages in modern UK history would be in deep trouble. However, for much of the media, the goal of macroeconomic policy has been redefined as how effective the government has been at reducing the deficit. Macroeconomics as portrayed by the media is so different from the macroeconomics of the textbooks that I call it “mediamacro”.

Nothing illustrates mediamacro better than Ed Miliband’s 2014 Labour conference speech, in which he forgot to mention the deficit. In terms of what influences national prosperity, the real news over the past five years has been the stagnation in UK productivity. Yet when David Cameron failed to mention the productivity slowdown in his conference speech, hardly any journalist bothered to highlight this huge omission. When Miliband forgot to mention the deficit even Jon Snow lambasted him.

How did the coalition government manage to transform the media debate on macroeconomic policy so comprehensively? I have some idea of the ingredients involved but much less idea of how important each is. Of course having a partisan press is important, if only because it is capable of setting agendas. It also helps that the BBC can be easily intimidated. When its former economics editor Stephanie Flanders dared suggest that a lack of productivity growth might be a problem, Iain Duncan Smith made a formal complaint.

There is a further problem with how the media generally get their economic expertise. The economists you are most likely to see in the media are those who work in the City. It is, after all, part of their job to get media exposure; they’re always on hand to give a reaction. To be fair, when it comes to the daily ups and downs of the market, they are also best qualified to play this role, though in fact no one knows why markets move from day to day. But on issues of macroeconomic policy, City economists can present a biased and distorted view.

At the beginning of 2014, the Financial Times conducted a survey of economists; one of the questions it asked was: “Has George Osborne’s ‘plan A’ been vindicated by the recovery?” As I have already suggested, this question has an obvious answer. The 2013 recovery could not possibly vindicate the 2010 austerity because it is exactly what you would have expected to happen after austerity initially reduced GDP growth and was eased as a result. Among the academics answering this question, there were ten clear nos and only two clear yeses. However, among the many City economists who answered the FT survey, the numbers of yes and no replies were more evenly balanced.

Granted, it is regrettable that academic economists cannot speak with complete unanimity on the matter, but a 2/10 split is as close to a consensus as these things go. It is also the case that almost all academic macroeconomists would argue that the cuts in public investment that occurred in 2010 were a grave mistake. As the New Statesman reported in 2012, many of the minority of economists who originally supported immediate austerity have since acknowledged that cutting public investment in 2010 and 2011 was a grave mistake. It was these cuts, such as halting repairs to schools or reducing spending on flood defences, which most damaged GDP.

The austerity mistake involves basic macroeconomics. Cutting spending will reduce demand and is not to be undertaken when interest rates cannot be cut to offset its impact. The Conservatives, if elected, plan further sharp austerity in the early years of the next parliament, at a time when interest rates are still expected to be at or near their floor. Whatever your views about the desirable size of the state in the long run, to cut spending when the economy is still vulnerable in this way is to take a huge risk. It is exactly the risk that materialised from 2010, except today there is not even a hint of market pressure to cut the deficit quickly. Being able to cover up the earlier mistake is bad enough. Planning to repeat it is pure folly.

Simon Wren-Lewis is a professor of economics at Oxford University