Humans’ and pigeons’ observed and predicted accuracy for prototypes, typical items, and exceptions during the early (top panels) and late (bottom panels) stages of learning within the prototype-exception task of Cook and Smith (2006). Observed performances are shown as filled, black circles. The best-fitting predictions of prototype and exemplar models are shown, respectively, as open squares and triangles. From “Stages of abstraction and exemplar memorization in pigeon category learning,” by R. G. Cook and J. D. Smith, 2006, Psychological Science, 17, p. 1059–1067. Copyright 2006 by the Association for Psychological Science. Reprinted with permission.