Come September, on our return from summer holidays, there is one certainty in life. As we wander round the supermarket we'll be met by the Christmas aisle - yes, already - rows of crackers, mince pie samples and the smell of pine or spice-scented candles.

Funnily, it's the candles that are likely to take us back to our childhood - hardly surprising, according to studies which show that 75% of our emotions are generated by what we smell rather than what we see or hear. Marketers have never been backwards about exploiting this, either through the product, its packaging, or both.

They may not understand the inner workings of the brain which makes consumers so receptive to smells - but that's not going to hold them back.

So why are we so susceptible to odours? According to Hy Mariampolski of QualiData Research Inc., "Olfactory memory is associated with deep emotional memories because they are all resident in the limbic system and its associated components such as the hippocampus and amygdala.

"These parts of the brain are connected with primal individual emotions such as self protection, maternal love, care of children, sexual desire and satisfaction of hunger. There is little consensus about the precise biochemical process by which these connections occur, and it is clear that olfactory memories may be inaccurate as well as based on context and subsequent experience. Nevertheless, they produce powerful emotional associations."

That's why we have grocery chains baking pre-prepared dough in store so as to generate the smell of freshly cooked bread and why house vendors are recommended to brew coffee before potential buyers arrive. It's even why some airlines bake cookies as the plane is landing so as to provide comfort to all on board (plus cover up any noxious odours that emerge).

And forget about rumours of gas canisters that spread those familiar freshly-baked bread and roasted coffee smells. According to Simon Harrop of The Aroma Company, these are perfumery Holy Grails that haven't been achieved accurately yet. "If we could replicate those smells we would be very wealthy, although we can achieve odours like cappuccino or cakes."

Still, retailers don't always get it right. Phillip Adcock is managing director of research agency and retail consultancy Shopping Behaviour Xplained. In a previous incarnation, he was a merchandising manager at Comet and spending, as he puts it, a fortune on research. What he needed, though, was data he could action.

His solution was to set up a company with a psychologist friend. "He did the psychoanalysis of people's behaviour in store, we discussed it, I turned it back into English and then sold it to clients," he says.

He describes one unfortunate project at Matalan. "Women's lingerie wasn't selling," he says. "There was nothing wrong with it, it was Playtex, a good brand, but it just wasn't selling. So we put some cameras in to see what was going wrong. What transpired to be the answer was a pile of bath mats nearby smelling of curry. "There was a classic incongruence there; you were seeing clean lingerie smelling of curry. Not a single interviewee told us what was wrong with the department, they didn't know. You just had to look at the body language."

The other side of the coin, he adds, is that you don't always need to introduce a smell if you can create the right visual stimuli to get people thinking it. This is sufficient, in many cases, to alter perceptions.

It's a complicated area, however, with many variables. Packaging, for instance, can leach aromas from product so manufacturers have been forced to look at remedies. "It is fair to say that the packaging can enhance the effect you get when you first open the product," says Simon. "If you have a bag of salt and vinegar crisps, say, you can take their flavourings and embed that into the packaging so that when you open it up you get an enhanced flavour experience."

There can be a gulf between the smell experience of a food and the taste experience, he adds, though when manufacturers tackle packaging issues it's not so much about over-promising but of matching consumers' expectations to the perceptions that they have when they eat the product.

Audi is an example of a brand that takes smells to the other extreme. The 'chief sniffer' at its offices in Ingolstadt near Munich heads a team who smell everything that goes into a new car's interior - but aims for complete neutrality. The rationale is that if smells are added to cars, consumers might not like them or complain when they fade. There again, since Audi sells in over 100 countries, there is also the need to ensure that the glues and plastics used in manufacturing don't pong in very hot climates.

Which is all very well for a brand whose manufacturer controls its selling environment, but pity the poor air freshener products sold in the next aisle to the bleach.