Children become emotionally attached to cuddly toys, blankets and even smelly old scraps of material because they intuitively believe they possess a unique essence or life force, psychologists said yesterday. In a study that will surprise few parents, it was found that children preferred their cherished comfort blankets or favourite raggedy bear over duplicates apparently identical in every way.

The results suggested that even very young children invest in such objects intangible qualities that cannot be replicated.

The study compared the reaction of the children to that of art enthusiasts who prefer an original to a copy that is identical in every way.

Previous studies have shown that up to 70% of young children develop strong attachments to objects such as toys or blankets. The phenomenon tends to be confined to the western world, where children usually sleep apart from their parents at an early age.

Bruce Hood, of the University of Bristol, and Paul Bloom of Yale University in the US, decided to try to find out why.

Parents were asked to bring children aged three to six into a laboratory with their "attachment object" or if they had no such object, a toy or doll that they liked.

To count as an attachment object, the child had to regularly sleep with it and have had it for at least a third of his or her life.

The children were shown what they were told was a "copying machine" - in reality a conjuror's cabinet made up of two boxes. The doors of the two boxes were open and a green block put into one of them. Then the doors shut, an experimenter twiddled some knobs and the first box buzzed.

A few moments later a buzz came from the second box. The doors of both boxes opened to reveal a green block in both of them - the experimenter had slipped an identical block into the second box.

Then the experimenter asked the children if they would let the objects they had brought in be copied. They could choose if they wanted the new one or the old one back. All of those with "non-attachment" objects allowed them to be copied and almost two-thirds decided to keep the "new" object - in fact, it was their own object.

Of the 22 children who did have attachment objects, four stubbornly refused to allow them to be copied at all. Of the 18 who did let their precious items be copied, only five opted to have the "duplicate".

At the end, all children were shown how the illusion worked so they knew they had their original item back.

Prof Hood said the experiment showed that children believe that in addition to the physical properties of their objects, there was some other quality to them that cannot be copied.

He said: "If there was a machine which copied a favourite object in every way down to atomic level, we would still prefer the original. It has an essence to it. This experiment suggests this is an intuitive process.

"We anthropomorphise objects, look at them almost as if they have feelings. The children know these objects are not alive but they believe in them as if they are."

While the tendency to sleep with a comfort blanket is thought to be largely a western trait, Prof Hood said believing objects had an essence was not. Some eastern beliefs centre on all things having a life force and some cultures find it difficult to live in other people's homes because they feel there is something intangible left of the previous people in them.