Are apes essentialists? Scope and limits of psychological essentialism in great apes.

Human reasoning is characterized by psychological essentialism (Gelman in The essential child: origins of essentialism in everyday thought. Oxford University Press, New York, 2003): when reasoning about objects, we distinguish between deep essential properties defining the object's kind and identity, and merely superficial features that can be changed without altering the object's identity. To date, it is unclear whether psychological essentialism is based on the acquisition of linguistic means (such as kind terms) and therefore uniquely human, or whether it is a more fundamental cognitive capacity which might be present also in the absence of language. In the present study, we addressed this question by testing whether, and if so, under which circumstances non-human apes also rely on psychological essentialism to identify objects. For this purpose, we adapted classical verbal transformation scenarios used in research on psychological essentialism (Keil in Concepts, kinds, and cognitive development. MIT Press, Cambridge, 1989) and implemented them in two nonverbal tasks: first, a box task, typically used to test object individuation (Experiment 1), and second, an object choice task, typically used to test object discrimination, object preferences and logical inferences (Experiments 2-4). Taken together, the results of the four experiments suggest that under suitable circumstances (when memory and other task demands are minimized), great apes engage in basic forms of essentialist reasoning. Psychological essentialism is thus possible also in the absence of language.