A profound disparity revisited: Perception and judgment of abstract identity relations by chimpanzees, human infants, and monkeys.

We review the evidence for three important disparities involving the perception and judgment of identity relations by human and nonhuman primates. First, only humans beyond infancy and adult chimpanzees (Pan troglodytes) with a history of language or token training can explicitly judge relations (same or different) between relations (identity and nonidentity) in a matching-to-sample (MTS) task. Second, both human and chimpanzee infants perceive relational similarity as measured in preference-for-novelty tasks. The human and chimpanzee infants, however, do not express this tacit knowledge in judgmental tasks like relational MTS. Third, unlike the human and chimpanzee infants, adult rhesus monkeys (Macaca mulatta) tested with the same preference-for-novelty tasks do not perceive abstract relational similarities and differences despite their sensitivity to physical identity.