A chimpanzee, a scientist with a stick and a researcher in a King Kong suit may sound like the setup for a bad joke, but it is in fact the basis of a recent study that provides the first evidence that great apes—that is, bonobos, chimpanzees and orangutans—possess an understanding of false belief, a hallmark of “theory of mind.” This ability to understand that others have mental states and perspectives different than our own has long been considered unique to humans.

In the study, published Thursday in Science, a team of scientists recorded the eye movements of three great ape species while the animals watched videos of a man searching for a hidden object that had been moved without his knowledge, and found that they looked more frequently at the location where the man expected the object to be (a belief the apes knew was false), even though the object was no longer there. The findings suggest the apes were able to intuit what the human was thinking.

Theory of mind is central to our social functioning as humans, but scientists have long wondered whether it is, in fact, a uniquely human trait. There is evidence that apes can understand other’s mental states when they match up with reality, but apes have consistently failed tests of false belief—the idea that someone else may act according to a belief that is untrue. Fumihiro Kano of Kyoto University’s Kumamoto Sanctuary and a co-leader of the study calls this ability a “litmus test” for theory of mind. Traditional false-belief tests for apes have involved complicated tasks such as moving around cups to reveal hidden food. This is why Kano and study co-leader Christopher Krupenye of the Max Planck Institute for Evolutionary Anthropology adapted a simpler false-belief test designed for human infants that utilizes an eye-tracking method called anticipatory looking, or gazing at where you expect a person to look for an object.

During the study, bonobos, chimpanzees and orangutans were “invited” one at a time to sit in a room and drink juice while watching a sequence of scenarios on a video monitor. An infrared camera below the monitor recorded where on the screen the animals were looking as they watched the scenes unfold. To capture the apes’ attention, the researchers made each experimental scenario into a high stakes television drama starring a mysterious apelike character (a researcher in a gorilla suit), whom they dubbed King Kong.

Like humans, great apes are “pretty obsessed with social information—when there's a conflict within their group everybody stops and pays attention,” Krupenye says. “This is just their version of the Jersey Shore that we made so they would be really engaged and curious about what was going to happen.”

In one scenario the King Kong figure pretended to attack a researcher, then hid in one of two hay bales, moving to the other bale while the researcher watched. Then the researcher left for awhile before returning with a stick to look for King Kong, who had left the scene while the researcher was away. In another scenario the costumed figure moved to the other hay bale after the researcher left and then departed entirely. The researchers also set up the same two scenarios in a slightly different setting—instead of hiding himself, King Kong hid a stolen rock under one of two boxes before removing it completely.

Apes from all three species consistently passed the test; even though the animals knew King Kong or the rock was gone, when the researcher returned to search for it, they consistently looked at the hay bale or box where the person had last seen the object and presumably still thought it was hidden. These results are particularly surprising because they challenge the large body of previous work that suggests great apes are not capable of comprehending beliefs that are untrue. “People have thought for awhile that false-belief understanding is unique to humans,” Krupenye says, “and so this suggests that apes do have at least a basic, implicit understanding of false belief, which has been seen as a signature of theory of mind.”

Their findings drew both praise and debate in the field. In an article about the study also published today in Science, Frans de Waal, a primatologist who studies social intelligence at Emory University and was not involved with the work, wrote the study design “is a genuine breakthrough, not only because it avoids an undue reliance on language skills required to understand narrative and questions in theory of mind testing in children, but also because it highlights the mental continuity between great apes and humans.”

Tecumseh Fitch, an evolutionary biologist and cognitive scientist at the University of Vienna, also not part of the research, sees this as the “final nail in the coffin of the long-standing idea that humans are the only species with ‘theory of mind.’”

Others are skeptical of that interpretation, however. Carla Krachun at the University of Saskatchewan and Robert Lurz at Brooklyn College, who both study theory of mind in primates, are excited that the researchers were able to indirectly measure apes’ mental processes using eye tracking, which “opens up all sorts of possibilities for studying theory of mind in apes,” they wrote in an e-mail. Krachun and Lurz do not think the study definitively demonstrates false-belief understanding, however. “The issue is that subjects could use a simple behavior rule—‘agents search for things where they last saw them’—to pass the tests without understanding anything about the agent’s false beliefs,” they explained.

Kato and Krupenye acknowledge the difficulty of interpreting their findings but still see them as an important step forward in our understanding of great ape cognition. “There are other kinds of false beliefs that I think we need to test in order to be sure that apes are relying on this more sophisticated skill,” Krupenye says. “But the big thing here is that the apes clearly have a more sophisticated understanding of others than we previously thought, and that means they can predict others' behavior even in contexts when the actor is misguided, and that's something that humans do all the time.”