One of the most persistent myths that passes for popular economic wisdom is that an economy is in an official recession when the data records two consecutive quarters of negative growth.

Nobody seems to know exactly where this arbitrary and patently rather silly idea originated.

One theory is that it was a definition dreamed up by advisers to President Johnson in the 1960s.

It was a clever wheeze, which enabled him to argue that, despite the gathering economic misery around him — rising unemployment and falling incomes — the fact that, through calendrical happenstance, there had not actually been two straight quarters of declining growth meant that “officially” everything was all right.

It is, on its face, a flawed definition. Suppose that an economy contracts by 5 per cent one quarter, grows by 0.1 per cent the next and contracts by another 3 per cent the next quarter.

From the beginning of the first quarter to the end of the third, the economy will have contracted by about 8 per cent, with accompanying sharp declines in employment and wages.

Yet “officially” there will have been no recession, according to everybody's favourite definition.

We will doubtless hear quite a bit about the “Two Quarters Doth a Recession Make” tomorrow when the US Commerce Department publishes its first estimate for the American economy's gross domestic product for the first quarter of 2008.

Despite the general impression that the United States has had an absolutely miserable start to the year and all that hyperventilating talk about the onset of another Great Depression, it is quite likely that Wednesday's figures will show that, in the first quarter, output actually expanded.

The mid-range of Wall Street economists' forecasts puts the likely GDP number at a growth rate of 0.4 to 0.7 per cent annualised.

This pretty anaemic number will be positive only because of a very strong performance from the external sector (strong export growth and weak import growth helped by the weak dollar) and a sharp, unplanned increase in companies' inventories, caused by the softness of consumer demand.

Final domestic sales — the recorded numbers for inflation, adjusted spending by US consumers, business and government — will, in all probability, be negative.

But a plus sign is still a plus sign. You might recall that in the fourth quarter of 2007, US GDP expanded at an annual rate of 0.6 per cent.

So, in the past six months there has been no “official” recession. And since the first quarter was positive, even if the current, second, quarter is negative, we won't be able to call “Recession!” for at least another six months.

All this semantic babbling obscures the deeper reality that America has clearly been in a period of serious weakness for the past six months.

As it happens, the United States does have something akin to an “official” definition of recession.

It is decided by something called the business cycle dating committee of the National Bureau of Economic Research, based in Cambridge, Massachusetts.

The dating committee is not a matchmaking service for lonely heart business cycle economists, but a group of scholars who pore over the contemporary data and try to pinpoint, to within a month rather than a quarter, when cycles begin and end.

We already know, whatever the outcome of their deliberation, that growth has been essentially flat throughout that period. Because of population and productivity growth, the US needs to expand at a rate of about 2.5 per cent just to keep unemployment from rising.

So the effect of flat growth is negative for the economy and almost everybody in it.

The real question now is not whether America can somehow, by a statistical fluke, avoid an official recession, but whether the current period of pronounced weakness will continue, whether it might actually get worse or whether the upturn is about to start.

The consensus of economists is that things are going to get worse in the immediate future but will bounce back later in the year. GDP in the current (second) quarter is expected to be negative, but the second half of the year is supposed to be positive.

However, the recent signals have been mixed. Clearly, the relative resilience of retail spending and industrial production has been encouraging.

Equity markets have just come off their best two-week stint this year amid rising optimism that the worst is past.

No one quite knows how much worse the housing market will get or whether it might even have stopped depressing the economy.

Housing has clearly undermined consumer confidence and demand over the past few months and even if prices have stopped falling (unlikely, at least if anecdotal evidence in the Washington area is a guide), it will be a long time before homeowners start to feel confident again about the state of their financial health.

The big worry now is the shock from energy and food prices. Financial markets may be healing, but the outlook is clouded by soaring oil and food costs.

The summer driving season is going to be very expensive, with petrol clocking in at $4 a gallon. Last week one of America's largest retailers actually started to ration purchases of rice.

Only when the battered American consumer has withstood these most un-American of horrors will we be able to say with confidence — official recession or no — that the worst is truly over.

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