Evidence that humans can categorize is typically obtained by verbal tests. In animals, however, evidence has been ambiguous because, in most cases, a correct answer could be explained by memory of particular features associated with the correct choice. Such explanations were ruled out by a new paradigm for studying categorization, one that presents novel exemplars of simultaneously presented categories on each trial, 4 in this instance, on a touch sensitive video monitor. Monkeys were required to respond to the categories in a prescribed order. Because reward was not delivered until a subject responded to all 4 exemplars in the correct order, accurate performance could not be explained by binary choices or by memory of particular features. Monkeys and human subjects responded on this paradigm at a much greater than chance accuracy on two categorization problems; one based on novel exemplars of photographic stimuli of natural objects (birds, cats, flowers and people), the other on novel exemplars of small sections of paintings by different artists (Dali, Gérôme, Monet and Van Gogh). These results provide the strongest evidence to date of categorization by an animal and also demonstrate a common classification strategy by monkeys and humans. In the case of humans, it did not appear that they used verbal labels to select the correct categories.
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