Many older people gradually develop amnesia about recent happenings while retaining a wealth of detail from their younger days. They may make up stories to cover their embarrassment about the blanks, and generally they know their memory is foggy. The kind of storytelling my grandmother did after a series of strokes is a little different. Neurologists call it confabulation. It isn't fibbing, as there is no intent to deceive and people seem to believe what they are saying. Until fairly recently it was seen simply as a neurological deficiency - a sign of something gone wrong. Now, however, it has become apparent that healthy people confabulate too.

Confabulation is clearly far more than a result of a deficit in our memory, says William Hirstein, a neurologist and philosopher at Elmhurst College in Chicago and author of a book on the subject entitled Brain Fiction (MIT Press, 2005). Children and many adults confabulate when pressed to talk about something they have no knowledge of, and people do it during and after hypnosis. This raises doubts about the accuracy of witness testimony (see "The unreliable witness"). In fact, we may all confabulate routinely as we try to rationalise decisions or justify opinions. Why do you love me? Why did you buy that outfit? Why did you choose that career? At the extreme, some experts argue that we can never be sure about what is actually real and so must confabulate all the time to try to make sense of the world around us.

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[Armin Schnider] believes that we must all have a pre-conscious brain mechanism that distinguishes between current reality and fantasy, or a memory that is no longer relevant. "The brain decides long before the thought becomes conscious," he says. His recent recordings using scalp electrodes show that when people see the pictures in a second run, yet correctly suppress memories irrelevant to this test, they show a characteristic pattern of brain activity after 0.2 to 0.3 seconds, whereas it takes twice that time for people to become consciously aware of the judgement. The decision process happens subconsciously, too early for awareness. Our brain sorts fact from fiction well before we know our own thoughts, he concludes.

So confabulation can result from an inability to recognise whether or not memories are relevant, real and current. But that's not the only time people make up stories, says Hirstein. He has found that those with delusions or false beliefs about their illnesses are among the most common confabulators. He thinks these cases reveal how we build up and interpret knowledge about ourselves and other people.

It is surprisingly common for stroke patients with paralysed limbs or even blindness to deny they have anything wrong with them, even if only for a couple of days after the event. They often make up elaborate tales to explain away their problems. One of Hirstein's patients, for example, had a paralysed arm, but believed it was normal, telling him that the dead arm lying in the bed beside her was not in fact her own. When he pointed out her wedding ring, she said with horror that someone had taken it. When asked to prove her arm was fine, by moving it, she made up an excuse about her arthritis being painful. It seems amazing that she could believe such an impossible story. Yet when Vilayanur Ramachandran of the University of California, San Diego, offered cash to patients with this kind of delusion, promising higher rewards for tasks they couldn't possibly do - such as clapping or changing a light bulb - and lower rewards for tasks they could, they would always attempt the high pay-off task, as if they genuinely had no idea they would fail.

One rare condition can make people confabulate even more elaborate tales. Capgras's syndrome sometimes affects people after a stroke, and can leave them believing that their loved ones have been substituted by identical-looking impostors, so they make up stories of alien abduction and conspiracy in an attempt to explain this crazy situation. In similarly strange conditions people may lose the ability to recognise themselves in the mirror, or may even believe they or another person are dead, despite all evidence to the contrary. In each instance, the affected person confabulates to explain the weirdness, oblivious to the absurdity.

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[Morten Kringelbach, a neuroscientist at the University of Oxford] goes even further. He suspects that confabulation is not just something people do when the system goes wrong. We may all do it routinely. Children need little encouragement to make up stories when asked to talk about something they know little about. Adults, too, can be persuaded to confabulate, as Timothy Wilson of the University of Virginia in Charlottesville and his colleague Richard Nisbett have shown. They laid out a display of four identical items of clothing and asked people to pick which they thought was the best quality. It is known that people tend to subconsciously prefer the rightmost object in a sequence if given no other choice criteria, and sure enough about four out of five participants did favour the garment on the right. Yet when asked why they made the choice they did, nobody gave position as a reason. It was always about the fineness of the weave, richer colour or superior texture. This suggests that while we may make our decisions subconsciously, we rationalise them in our consciousness, and the way we do so may be pure fiction, or confabulation.

More recent experiments by philosopher Lars Hall of Lund University in Sweden develop this idea further. People were shown pairs of cards with pictures of faces on them and asked to choose the most attractive. Unbeknown to the subject, the person showing the cards was a magician and routinely swapped the chosen card for the rejected one. The subject was then asked why they picked this face. Often the swap went completely unnoticed, and the subjects came up with elaborate explanations about hair colour, the look of the eyes or the assumed personality of the substituted face. Clearly people routinely confabulate under conditions where they cannot know why they made a particular choice. Might confabulation be as routine in justifying our everyday choices?

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Even when we think we are making rational choices and decisions, this may be illusory too. The intriguing possibility is that we simply do not have access to all of the unconscious information on which we base our decisions, so we create fictions upon which to rationalise them, says Kringelbach. That may well be a good thing, he adds. If we were aware of how we made every choice we would never get anything done - we cannot hold that much information in our consciousness. Wilson backs up this idea with some numbers: he says our senses may take in more than 11 million pieces of information each second, whereas even the most liberal estimates suggest that we are conscious of just 40 of these.

Nevertheless it is an unsettling thought that perhaps all our conscious mind ever does is dream up stories in an attempt to make sense of our world. "The possibility is left open that in the most extreme case all of the people may confabulate all of the time," says Hall.