Credit: S. STECKENFINGER

Proc. Natl Acad. Sci. USA doi:10.1073/pnas.0910063106 (2009)

As robots and computer-generated avatars grow more like humans in appearance, real humans become more accepting of them, although only to a point. When non-human figures start looking too much like humans, people react negatively — an effect called the 'uncanny valley'.

Now Shawn Steckenfinger and Asif Ghazanfar of Princeton University in New Jersey have shown that the uncanny valley is not unique to humans. They presented five macaques (Macaca fascicularis) with video footage and static images of real monkey faces along with realistic and unrealistic synthetic faces (pictured). The macaques preferentially looked at the real faces and unrealistic synthetic ones, shunning the realistic synthetic ones.

As the effect is apparently not dependent on human culture or brain structures, an evolutionary root for this is plausible, the researchers say.