The moment when a shopper decides to buy can now be clearly seen on a brain scan, thanks to a US study.
Volunteers were scanned as they looked at desirable goods
Volunteers were scanned as they looked at desirable goods and their prices, and the most active brain areas recorded.
The results suggest different parts of the brain anticipate having the item, and feel the 'pain' of preparing to part with money.
The Stanford University study was published in the Neuron journal.
The experiment used an MRI scanner set up to detect brain activity.
Deal or no deal
Pictures of goods such as boxes of chocolates were shown, then a few seconds later, the price flashed up on the screen, before the volunteers were asked to decide whether or not to buy.
The timing meant that the mental battle between desire for the object, and caution about spending, was clearly shown.
If the volunteer wanted the product, a part of the brain called the nucleus accumbens lit up.
Then, when the price appeared, a high price activated a part called the insula, and deactivated other parts.
In this way, the researchers were able to predict the outcome.
The researchers said the results clearly demonstrated the truth of the principle that purchasing is a mental 'trade-off' between the pleasure of acquisition and the pain of paying.
They said that the study could reveal how credit cards 'trick' the brain into buying more.
They wrote: "This finding has implications for understanding behavioural anomalies, such as consumers' growing tendency to overspend and under save when purchasing with credit cards rather than cash."
Dr Alain Dagher, from the Montreal Neurological Institute, said that the brain mechanisms in action were evolved to suit more primitive uses.
He said: "Human financial behaviour is often seemingly irrational, a fact that provides employment for advertisers, casino workers, insurance salesmen, and economists."
He said that it might be possible to understand more about compulsive shoppers and gamblers by spotting problems in the brain areas revealed by the scans.