Evidence of psychological essentialism in a symbol-trained bonobo ( Pan paniscus )

Psychological essentialism is the ability to recognize that the class or kind to which an object belongs remains unaltered despite changes to its superficial perceptual features. It is unclear whether humans are unique in possessing this ability or whether it is present in nonhuman primates. A recent study with three great ape species found suggestive evidence of psychological essentialism, but further research is needed to rule out the alternative explanation that the apes used locational information to pass the test. We tested a bonobo ( Pan paniscus ) with specialized symbolic skills, Kanzi, using a modified procedure that avoided this alternative explanation. Kanzi passed the test, which required him to indicate which of two superficially identical but essentially different kinds of items was actually the item requested by the experimenter (e.g., successfully indicating the banana when both items looked like carrots). A control test indicated that he had not passed the essentialism test simply by detecting differences in the superficial appearance of the items. Our findings support the conclusion that great apes are capable of psychological essentialism; however, Kanzi is unique in that he is able to comprehend some simple spoken English and use lexigram symbols to communicate with humans. The extent to which these skills contributed to his essentialism is unknown. Further research designed to specify the limits of essentialism in bonobos, and other great apes, that are not symbol-proficient can inform debates about the phylogenetics of essentialism and the role of language in its development and expression.