A raggedy blanket, a tatty teddy bear: the dog-eared appearance of many childhood possessions is testament to how dearly they are held. But when and how does this sense of ownership begin?

Even a newborn regards their mother as "special", and will seek out her face and smell over those of other women. By two months, babies begin to understand that they have ownership of their own bodies, while at eight months they start to grasp the concept of loss. By 12 months, they start to form attachments to comfort-objects, such as blankets. Psychologists suggest these provide a temporary substitute for their caregiver.

Also around one year, children start to say their first words, usually nouns such as "bath" and "duck". By 21 months or so a word surfaces that will provide a soundtrack for the coming years: "mine".

Not for nothing are they called the terrible twos: the constant squabbles over possessions are combined with an underdeveloped sense of empathy and a propensity to tantrum. Two-year-olds fight harder for toys when they actually own them, indicating they can distinguish temporary possession from longer-term ownership, says Susan Gelman, who studies conceptual development in children at the University of Michigan in Ann Arbor, in the United States. "By three years of age they even protest if someone tries to take or throw away someone else's toy, which shows that they understand ownership even when it doesn't involve their own self-interest."

Children's concept of ownership continues to change as they grow older. Gelman's team recently ran an experiment in which two- and three-year-olds were shown three objects; one they were told was "theirs", one which belonged to the researcher and one simply placed beside the others. When the items looked different, two-year-olds had no problem identifying which was theirs, but if they were identical, or their object was less desirable, they would become confused. In contrast, three-year-olds kept track - even when their object was far less desirable than the other two.

This may help to explain why the replacement of a lost "blankie" or teddy bear with a newer model never goes down well: ownership overrides appearance. Indeed, when Bruce Hood, from Britain's University of Bristol, showed three- to six-year-olds a "magic copying machine" that could replicate their favourite toy, most children demanded the original back, and a quarter refused to have it copied at all. Ownership seems to bestow a magical quality that can't be faked - even in young children.

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