People are particular about their things. Property—who owns it or did what with it—is the subject of many a legal battle. It’s odd to me how attached people get to objects and how emotional they become when someone messes with their stuff. Yet we take notions about sharing and rules such as “don’t take what is not yours” very seriously—so much so that grownups get upset when kids seem to flout them. So it might help to realize that an understanding of ownership has its own developmental trajectory, one that psychologists are just beginning to work out.

Obviously, a baby doesn’t care who owns the rattle she is shaking—or the dress she is wearing. But babies don’t express any opinions at all, really, and it’s obvious when they chew on your book, that they mean no disrespect. By age two, suddenly, that baby christens everything in the room “Mine!” But although she seems to have an idea of things belonging to her, she cannot fathom the possibility of an object belonging to someone else. So by default, she decides everything is hers. She isn’t selfish; in her mind, there can be no other possibility. At the age of three, she may no longer be the owner of all that she sees. But she is hardly wise in the ways of stuff. If she had a hand in making something, it is unequivocally hers, no matter that the material used to construct the masterpiece was not. Adults take a more nuanced view. (See “Passion for Possessions: Mine!” By Bruce Hood, Scientific American Mind, September/October 2011.)

Although we may find a toddler’s ideas of property rights markedly primitive, some research suggests that kids don’t act like adults with regard to objects before about age 10. When evaluating scenarios about other people, younger children tested in a study did not see any ethical difference between destroying something you own (which is okay) and ruining an item that belongs to someone else (wrong). (I have also noticed a change around age 10 in a child’s regard for small toys given out at arcades and birthday parties. My eight year-old still covets these items; the ten-year-old can’t be bothered. What this difference says about the maturation of ownership concepts in the brain, I can't be sure. )

A new study published this month suggests, however, that kids start to adopt certain adult notions of ownership long before age 10. The findings show that a magical mental leap with regard to objects seems to occur at the astoundingly early age of three. Unlike two year olds, three-year-olds realize on some level that what is okay to do with an object can depend on who owns it, if the kids observe the action (rather than just hearing about it). In the journal Cognition, psychologist Michael Tomasello at the Max-Plank-Institute for Evolutionary Anthropology in Leipzig, Germany and his colleagues showed 30 three-year-olds and 30 two-year-olds a scenario in which a puppet took away, and then disposed of, a hat. In some cases, the hat belonged to the toddler. In others, the hat belonged to the puppet. And in the third scenario, the hat belonged to a third party.

The puppet took possession of the various hats and happily tossed them in the trash, right in front of the kid. The researchers then watched how each child reacted. Tossing your own hat in the trash is acceptable. We might think it’s a waste of a perfectly good hat, but it’s allowed. Tossing someone else's hat in the trash, however, is not okay, according to ownership rules. The researchers expected a toddler to get upset when the puppet took and disposed of his or her own hat. What interested them was: Would he or she protest when the puppet threw away another’s hat?

The two-year-olds didn’t. They did not stand up for the property rights of the other person, although they did protest fairly often when the hat was their own. The authors think the two-year-olds did not understand the rule, but perhaps they did and just didn’t care enough to intervene. The three-year-olds, by contrast, did object when the puppet either took—or threatened to throw away—the other person’s hat. They said things such as “You can’t do that. It’s hers.” They complained far more in this scenario than when the puppet did the same to the puppet’s own hat. The study shows that kids begin to have a sense of right and wrong in relation to ownership at an earlier age than people had imagined. In particular, it demonstrates that three-year-olds know that it is okay, morally anyway, to throw away your own stuff (even if it is wasteful), but not to throw away someone else’s. “At three years of age but not younger, children start to appreciate other people's ownership of objects,” says psychologist Bruce Hood of the University of Bristol in England.

The work is a small piece in the emerging puzzle of how children’s concept of property develops. Kids might first learn that possession can take on different forms, the authors suggest. So they might discover early on, for instance, that being given something is different from borrowing it. Children might also get some idea about what true ownership implies—for example, that you have more say in what to do with something if it is officially yours. Later, kids may grapple with complexities. Although researchers are not sure which specific concepts kids get first, three-year-olds are already showing some understanding that property rules are made by the larger social group and that these social norms are important. They will even enforce these larger group rules whenever individuals break them.

The social perspective gives me a better sense of what these kids are learning about property. Some researchers say our feelings about objects stem from the fact that we consider things to be extensions of ourselves. But I find it easier to get my mind around this idea if there's a social context. To me, people are so much more important than things. When social rules guide our relationships toward objects and what we are allowed to do with them, then breaking the rules puts holes in the social contract—and the act becomes personal. When someone gives something to me, if that person is special, the object becomes special. So to me it makes sense if the development of the property parts of our brains is tied to the social parts. It may take a while to learn the social code, but perhaps it is that code that gives objects their emotional significance. And maybe my 10-year-old dismisses the plastic sunglasses and rubber balls in goodie bags because by now he knows that those gifts have become so obligatory that they carry scant social meaning. Or maybe he just remembers that he has not found them particularly useful.