Monkeys are freaked out by almost-but-not-quite-real depictions of themselves. That tendency is well documented in humans, but has never before been seen in another species.

To test their preference, researchers showed macaque monkeys real pictures, digital caricatures and realistic reconstructions of other monkey faces. To the latter, the macaques repeatedly averted their eyes.

"The visual behavior of the monkeys falls into the uncanny valley just the same as human visual behavior," wrote Princeton University evolutionary biologists Shawn Steckinfinger and Asif Ghazanfar in a paper published Monday in the Proceedings of the National Academy of Sciences.

The "uncanny valley" was identified in 1970 by Japanese roboticist Masahito Mori, who noticed that people presented with likenesses of increasing realism respond with increasing empathy, right up to the point where the likenesses are almost real. At that point, people are repulsed. The sudden dip in graphs describing their response gave the phenomenon its name.

Many explanations have been suggested for the uncanny valley, which has also been blamed for the box-office failure of movies like Beowulf and Final Fantasy. Perhaps almost-real humans look a bit too much like corpses for our comfort; perhaps they're so real that they engage our brains' mate-recognition or disease-avoidance systems, which promptly identify poor partners or sick individuals.

The PNAS results don't favor any one of these explanations, but do suggest that the uncanny valley has evolutionary origins deep in the primate psyche.

It remains to be seen how the monkeys would react to a simian version of The Polar Express.

Images: PNAS

See Also:

Citation: "Monkey visual behavior falls into the uncanny valley." By S. Steckenfinger & A Ghazanfar. Proceedings of the National Academy of Sciences, Vol 106. No. 40, October 12, 2009.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecosystem and planetary tipping points.