Public Release: 

Apes demonstrate human-like understanding of what others believe

American Association for the Advancement of Science

Apes can correctly anticipate that humans will look for a hidden item in a specific location, even if the apes know that item is no longer there, a new study reveals. The results, which show that apes can grasp what others know even when it differs from their own knowledge, demonstrate that nonhuman primates can recognize others' beliefs, desires, and intentions, a phenomenon called "theory of mind" (ToM), and one that has generally been believed as unique to humans. Several behavioral experiments to date have suggested that apes can predict others' behavior not simply based on external cues, but also on an understanding of others' perceptions and knowledge. Yet, it had been unclear whether apes grasp more complex mental states related to ToM, such as false beliefs. Here, Christopher Krupenye and colleagues studied three different species of ape - chimpanzees, bonobos, and orangutans - as they watched short videos on a monitor, tracking their gazes noninvasively using an infrared eye-tracker. The apes observed as a human witnessed the hiding of an object in one location. Next, the object was moved by an actor in a King Kong suit to a second location while the person was either present or absent. In both conditions, the object was then completely removed before the person returned to search for it. In the scenario where the human did not observe the relocation of the object, eye-tracking data reveals that 17 out of 22 apes correctly anticipated that the human would go to the incorrect location to search for the object. The apes' correct anticipation of where the human expected the object to be suggests that they understand that person's perspective. In a related commentary, Frans B. M. de Waal notes that "this nonverbal paradigm 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."

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