Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

David Washburn

Second Advisor

Sarah Barber

Third Advisor

Michael Beran

Fourth Advisor

Sarah Brosnan


Humans and nonhuman animals categorize the natural world, and their behaviors can reveal how they use the stimulus information they encounter in service of these categorizations. Rigorous psychological study of categorization has offered many insights into the processes of categorization and their relative strengths and weaknesses across species. Probabilistic categorization, in which the relationships among stimulus information and category membership that are observed by an individual are fundamentally probabilistic, presents unique challenges both to the categorizer and to the psychologist attempting to model their behavior. Challenges notwithstanding, probabilistic categorization is an exceptionally ecologically relevant problem to human and nonhuman animal cognition alike. This dissertation reports the effects of many manipulations of theoretical interest on computer-trained rhesus macaques’ and capuchin monkeys’ inferred cognitive strategy use in a computerized version of a classic probabilistic categorization task. Experiment 1 probed cognitive strategy use across five variants of the same task in which the probability structure was constant, but the appearances and onscreen locations of cues and responses changed. Experiment 2 presented a series of manipulations of theoretical interest to the animals by changing the probability and reward structures of the task. Experiment 3 manipulated the stimuli of the task in ways motivated by findings across perceptual psychology literature. Experiment 4 extended the reward rate manipulations of Experiment 2 even further. Across four experiments, inferred strategy use was remarkably stable. Those animals that used cue-based strategies often returned to the same specific strategy experiment after experiment, as the cues, responses, probabilities, and contingencies changed around them. This finding is discussed in relation to questions of a real or functional ceiling on sophistication of strategy use, the robustness of cognitive individual differences in nonhuman primates, and future directions for comparative study of cognitive strategy use in probabilistic categorization.

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