We all know that even if someone believes something, that thing may not in fact be true. This ability, to realize that other people are capable of holding erroneous beliefs, was once thought to be unique to humans. According to new research, however, some of our fellow apes may likewise be aware that what people think doesn't necessarily mesh with reality.

The study was conducted on captive chimpanzees, bonobos and orangutans, by scientists from Duke University, Kyoto University, the University of St. Andrews and the Max Planck Institute for Evolutionary Anthropology.

Each of the apes were shown two short videos.

In one, a person in a King Kong costume hides in one of two haystacks while a man watches. That man temporarily leaves through a door, at which point Kong secretly gets out of the haystack and runs away. The man then comes back, and tries to find Kong in the hay. In the second video, Kong places a stone in one of two boxes while the man watches, but then secretly steals it back when the man leaves – the man then comes back and goes to retrieve the stone from the box that he thinks it's still in.

Using infrared eye-tracking hardware set up outside the apes' enclosures, the scientists were able to tell what part of the screen the animals were looking at, at specific points in the videos. It was observed that as the man returned, the apes' gaze went to the haystack or box that they knew he would mistakenly approach – even though they themselves had seen Kong leave, and steal back the stone.

According to the researchers, the apes' performance was on par with that of human infants under the age of two who took part in a similar exercise. It is reportedly the first time that non-human animals have passed such a false belief test.

"If future experiments confirm these findings, they could lead scientists to rethink how deeply apes understand each other," said Duke University's Christopher Krupenye, who led the study along with psychologist Fumihiro Kano of Kyoto University.