Every year, the psychologist Tim Denison witnesses the same phenomenon - hordes of hyper-stimulated shoppers mesmerised into wasting millions on bargains that plainly aren't bargains. All thanks to the Christmas sales sending parts of our brains crazy. “Your perceptions can get wildly distorted,” he cautions.

Sales. The S-word is everywhere on our crunch-stricken high streets, and as we gear up for the traditional January bonanza, one thing is certain: many of those must-buys will turn out to be embarrassing blunders, thanks to the way our poor brains get bamboozled and bedazzled by the quick chance of a bargain. But there are scientific ways to safeguard your primitive grab'n'run instincts to help to ensure that precious cash is spent wisely rather than squandered.

Even in these days of cheaply available goods and seen-it-all-before shoppers, sales still spark our famine-fearing, feeding-frenzied instincts. Last month in America a WalMart employee was trampled to death by rampaging bargain-hunters. And in 2005, the prospect of cheap sofas sparked a riot at Ikea in North London.

Resistance is futile

We are all prone to this kind of panicked irrationality, says Denison, a retail psychologist at the global shopping-research company Synovate. “In sales the prices that people pay for things goes out of the window. They are not interested in the price so much as the ‘bargain',” he says. “There's a sense of getting rushed. You get mesmerised and assume that everything is far cheaper than it would be otherwise. You are swayed by the ‘75 per cent off' label rather than the price that you are paying.” He adds: “Another reason for overspending is the fact that your perception of cost savings gets wildly distorted. If you suddenly become convinced that you really want something, you may go for a 5 per cent reduction, whereas if you don't feel that bothered, you won't be moved by anything less than 60 per cent.”

Women are better bargain-hunters

Men and women can be as bad as each other in the sales, though in different ways, Denison says. Men tend to be poorer at working out what the actual market price of things is, not least because they are generally less interested in shopping. Women, meanwhile, “tend to be better at recognising genuine bargains but are more prone to getting wound up by the razzamataz”, Denison says. “They tend to get fixated on how much they have saved, rather than how much they've ended up spending.”

It's one reason why Anne Lord, a Brighton mother of two, always takes a notebook with her to the sales. “I walk around and if I see something I like, I make a note of what it is and where it is located. When I have finished my trawl of the shops, I go back around. If it's still there, and I still feel that I want it, I'll buy it. Often when I see it again, I'll decide that it's not so special. And if it's gone, well, given the way things are going, there will probably be another on sale cheaper in a few months' time.”

Lord's strategy works because it is a way of being mindful in a situation that often precipitates mindlessness. “We know that roughly 90per cent of our consumer behaviour is unconscious,” says Martin Lindstrom, an international branding consultant and author of the brain science book Buyology: How Everything We Believe About Why We Buy is Wrong. And that's on a good day.

Ancient fears urge us to grab

In the heat of a sales rush, “limited offers” create something called First World angst in our heads, when our ancient fears of scarcity and famine urge us to grab, grab, grab.

When we see crowds rushing after something, our primitive feasting response kicks in; we don't want the table to be bare when we get there. We also get ramped up on anxiety because we can see and even smell retail-frenzied anxiousness in others around us at an instant, subconscious level. This old instinct evolved so that we could detect if one of our tribe had spotted something nasty lurking in the grass, but now this anxiety contagion makes us want to grab even more - thanks to an ancient instinct to hoard things when faced with possible threats.

All this helps to explain why one study found that the stress levels of January sales shoppers are similar to those experienced by military pilots approaching a combat zone. Dr David Lewis, a cognitive neuropsychologist and co-author of The Soul of the New Consumer, says that his analysis of a survey of bargain-hunters shows that “crowds, rivalry between shoppers and determination to get that elusive bargain all combine to set hearts racing, blood pressure rising and, in extreme cases, cause full-blown panic attacks”.

“Binge-buying” gives us a chemical kick

Seeing and handling something we want to buy gives our brains a burst of the neurotransmitter dopamine, a reward chemical that is similar to heroin. The chemical high doesn't last long but you can stay high serially if you feel that the sales give you “permission” to keep buying and buying. In these straitened times, there is even social competition involved at the sales, in the shape of “spenvy” - discomfort caused by believing that your friends or family are actually better at snagging bargains than you. A survey of 2,000 shoppers for YouGov by Corinne Sweet, a consumer psychologist, showed that 78 per cent of us felt secret disappointment and guilt when others made their money stretch farther.

Paradoxically, this can make us rush out and spend harder, although ultimately less wisely.

Devise a shopping strategy

Anne Lord's method is one of several scientifically proven strategies you can adopt to help to ensure that you leave the shops with crunch-beating bargains rather than costly blunders. Researchers generally agree that it takes as little as 2.5 seconds to make a purchasing decision. So you need to take a break from the instant-grab reaction - and how better to distract yourself than with more browsing? If you find something you like, walk away and give yourself ten minutes to gain perspective, while examining other items. Even baboons use this sort of approach, says research by St Andrews University: on their gathering forays, they leave so-so foods untouched while they venture farther in search of top-quality items. If they can't find the best foods, they gather the second best on the way home.

Beware seductive Christmas colours

Retailers tend to snag our attention by using red and white labels for January sales items, as they are associated with Christmas and good times, says Denison. It works, as shops that have tried using other colours have found to their regret. Don't mistake those colours as automatically meaning bargain territory.

Take cash - and save money

Studies have shown that paying with cash at the till is much more painful than having your credit card swiped. Handing over money stimulates regions in your brain associated with discomfort. Credit cards seem to anaesthetise this.

Bring your husband or boyfriend

Men's stress levels soar during a hectic shopping spree. Studies indicate that men and women can shop together for about 70 minutes before they start to row. Men will have had enough, while most women will want to continue shopping for another half an hour.

John Naish is the author of Enough: Breaking Free From the World of More (Hodder & Stoughton)

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