Ever help a friend move and find yourself asking, why is she keeping that? A new study may have the answer.

When people were asked to choose between an iPod and, say, $100, they were more likely to choose the money. But when they were given an iPod and then immediately asked if they would like to trade it for $100, they were more likely to leave the Ben Franklin on the table.

This is called the endowment effect, which researcher Brian Knutson of Stanford University calls "the poster child of strange (economic) behavior." Chimps exhibit the effect, too.

Watching the subjects' brains with an fMRI (neuroimaging) machine, Knutson and his colleagues found that activity in the nucleus accumbens, which signals how much we like an object, did not increase when the new iPod belonged to the subject. But the right insular, which warns us about possible loss, became more active when the iPod became my iPod.

The results are detailed in the current issue of the journal Neuron.

While this suggests we are more motivated to protect our belongings out of fear than enjoyment, ownership may still have some intrinsic value. If you are particularly attached to something — say that battered college T-shirt or your wedding ring — the endowment effect is intensified. It hasn't yet been studied but Knutson predicts, in some cases of long-term ownership, there may be enhanced enjoyment as well as enhanced fear.

Why are we afraid of losing stuff? It's not like we will (really) die without our iPod.

But our brains may think we will.

The human brain is "ancient machinery in a modern time," Knutson explained. In our evolutionary beginnings, our possessions were basically limited to territory and kin — "belongings" critical to species survival and worth fighting to protect. This out-dated brain circuitry could be causing us to over-react when a sweater goes missing or a vase breaks. And prompting us to stand idly by as our homes become overrun with accumulated stuff.

Luckily, we can get better at assessing the true worth of our possessions. Experienced sellers, for example, are less victimized by the endowment effect. Knutson advised engaging in simulated markets to improve evaluation skills or asking someone else to appraise your belongings.

In other words, next time you move, invite a friend over to help you not pack.