But according to a study published Thursday in Current Biology, that mechanism -- which researchers have dubbed the "continuity field"-- can also steer us wrong, and have us convinced that two totally different faces or forms are the same.

"The brain is creating stability out of what's actually a very unstable system," said David Whitney, the senior study author and a University of California at Berkeley professor of psychology. His lab coined the continuity field term in a previous experiment. In that study, they observed the mechanism by which people meld similar looking objects together.

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"When you're watching Harry Potter, you don't notice that his plain T-shirt changes to a Henley, for example," first author and doctoral candidate Alina Liberman said. "Your visual system is primed to see things as remaining stable. You have a bias towards ignoring small changes in your environment."

This new study found the same bias in faces, which are obviously more complex -- and more important -- than T-shirts.

Liberman showed her subjects faces -- one every six seconds -- that varied in similarity to each other. After a brief shot of the face, they'd be shown a series of faces that got closer and closer to the one they'd just seen. But the subjects favored morphed faces halfway between their two previous target faces over the previously seen face on its own.

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In other words, the subjects were primed to believe that the two faces they'd seen at such close intervals had been the same person, and were quick to "recognize" a morphed face that supported that perception. So we don't just ignore Harry Potter's obvious wardrobe goofs -- we ignore the brief appearances of his stunt double, too.

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"If you see two faces in the span of about 10 seconds," Liberman said, "You expect them to look the same."

And thank goodness we do. "It's a way of smoothing our perception of people," Whitney said. "Without this mechanism, we'd experience fluctuations in identity." We'd have trouble recognizing someone as a single individual when they moved their face or moved into a different light.