Categorization in monkeys and chimpanzees

Studies on the classificatory ability of non-human primates are reviewed. The evidence suggests that there are important differences in the degree to which monkeys and chimpanzees detect same/different relations and construct classes that embody these relations. Data on matching to sample performance suggest that monkeys have a limited capacity for abstract representation of identity relations between individual objects. Chimpanzees, by contrast, not only detect similarities and differences between individual objects at a more abstract level than monkeys, but can also perceive same/different relations between pairs of objects. Further evidence for such cognitive differences between monkeys and chimpanzees comes from data on the development of spontaneous classificatory behavior. Monkeys develop first-order classifying. Their spontaneous spatial groupings are restricted to elements from one class. Chimpanzees progress from first-order to elementary second-order classifying. At 5 years of age they are capable of composing two contemporaneous sets in which objects are identical or similar within each set and different between sets.