The flop of the 2004 animated film The Polar Express is largely blamed on the “creepy” feeling people get when they look at very realistic-looking robots or human animations. These too real facsimiles fall into the so-called uncanny valley, between acceptably fake-looking human representations and real, healthy humans. Psychologists have long wondered whether this aversion has an evolutionary basis, and new research on macaques suggests that it does.

Princeton University researchers presented images of real monkey faces, unrealistic animated faces and realistic animated faces to five monkey subjects and recorded how long they gazed at each. Similar to the human response to objects in the uncanny valley, the monkeys avoided looking at the most realistic animated faces. The scientists, who published their results in the Proceedings of the National Academy of Sciences USA, speculate that realistic animations might resemble sickly or diseased animals because they lack subtle cues of health such as normal skin texture and hue—and that an aversion to such sights may have evolved to keep us healthy.