The learning of exclusive-or categories by monkeys (Macaca mulatta) and humans (Homo sapiens).

A central question in categorization research concerns the categories that animals and humans learn naturally and well. Here, the authors examined monkeys' (Macaca mulatta) and humans' (Homo sapiens) learning of the important class of exclusive-or (XOR) categories. Both species exhibited--through a sustained level of ongoing errors--substantial difficulty learning XOR category tasks at 3 stimulus dimensionalities. Clearly, both species brought a linear-separability constraint to XOR category learning. This constraint illuminates the primate category-learning system from which that of humans arose, and it has theoretical implications concerning the evolution of cognitive systems for categorization. The present data also clarify the role of exemplar-specific processes in fully explaining XOR category learning, and suggest that humans sometimes overcome their linear-separability constraint through the use of language and verbalization.