To the average person, bamboozled by talk of current-account deficits, exchange rates and bond yields, consumer confidence seems the sort of economics anyone can grasp.

Yet few indicators are as potentially deceptive - or as economically crucial.

But how can a collapse in the confidence of the average shopper have such a big impact on the economic fortune of the nation?

Shopping: Your patriotic duty

Consumer spending is certainly the foundation of many economies.

The long boom of the mid to late 1990s was built on buoyant spending - especially in the US and UK, where service industries have long replaced manufacturing as the main economic motor.

The best way to support the economy

Similarly, a slump in consumer spending is seen as the main threat to ensuring the US economy doesn't tip back into recession.

Policy-makers, especially in the US, have queued up to urge consumers to get back to the shops.

This vital economic role has pushed the various indicators of consumer confidence into a leading position among investors' numbers to watch.

In the US, every blip and dip of the University of Michigan's monthly consumer survey is chewed over obsessively by market pundits.

Lies, damned lies and statistics

But just how important are these confidence numbers?

Although no one questions the statistical rigour of the Michigan survey, or of its equivalents elsewhere in the world, experts counsel caution when jumping to economic conclusions.

Consumer behaviour is like an oil tanker: it can take a very long time to turn around

Peter Lunt, University College London

First, consumer confidence is what economists call a lagging, rather than leading indicator.

"Consumer behaviour is like an oil tanker: it can take a very long time to turn around," says Peter Lunt, a senior lecturer in economic psychology at University College London.

That is partly because consumers do not necessarily have the full range of economic information, either out of ignorance or because they are sceptical of what they hear from the media.

And the confidence lag is also a function of the fact that consumers cannot turn their economic habits around overnight.

Most consumers ramp up their debt during a boom period, for example, and are rarely nimble enough to pay it off quickly when the climate takes a turn for the worse.

Things change

And the role of consumer confidence may also have changed since it first started to be calculated in the 1950s.

Shopping was very different then

Then, the key psychological element in confidence was stability, especially in employment and welfare provision.

Now, Mr Lunt says many younger consumers are happy to have a little instability, because it offers possibilities as well as drawbacks.

An unstable labour market, for example, may mean people fear being sacked, but it also offers the chance for profitable job-hopping.

This increase in "risk-seeking" behaviour, economists say, means that unconfident consumers are not necessarily unhappy ones.

And while people may well be better-informed these days about general economic issues, they are also generally less concerned about society as a whole.

"As the old collective notions give way to more nuanced, individual thinking, the whole issue of confidence is much harder to predict," says Mr Lunt.

Spending differently, not less

And the reaction of gloomy consumers is not entirely predictable either.

"A drop in consumer confidence almost never causes a simple drop in consumer spending," says Gerrit Antonides, professor of economic psychology at Erasmus University in Rotterdam.

What it does produce, says Mr Antonides, is a fall in spending on durables; consumers put off upgrading their car, or buying a new washing machine.

Buying a new car may no longer seem so tempting

At the same time, they do not take on heavy levels of credit, and at least start to think about ramping up their savings.

Other goods benefit, notably "comfort products" - everything from chocolate and alcohol all the way through to books and home furnishings.

Overall, the amount of money passing through cash tills may not change that much - a fact that makes the wholesale sell-off of consumer-oriented shares seem something of an over-reaction.

Indeed, a little judicious spending, psychologists reckon, can be a great comfort to consumers at a time of stress.

Retail therapy may not just be good for the economy, it could be good for you too.