How Animals Classify Friends and Foes

A model of stimulus equivalence, which describes how non-similarity-based categories are formed, is used to describe aspects of animal social and communicative interactions such as kinship, friendship, coalitions, territorial behavior, and referential calling. Although this model was originally designed to deal with stimulus relations in linguistic behavior, it can be readily applied to understanding the cognitive mechanisms that underlie social as well as nonsocial categorizations in numerous taxa. This approach provides a new, parsimonious, and experimentally based understanding of how animals without language deal with problems of classification in their environment.