Running head: OCKHAM’S RAZOR AND PERCEPTION Perceptual Judgment Obeys Ockham’s Razor

Theoretical models of unsupervised category learning postulate that humans “invent” categories to accommodate new patterns, but tend to group stimuli into a small number of categories. This “Ockham’s razor” principle is motivated by normative rules of statistical inference. If categories influence perception, then one should find effects of category invention on simple perceptual judgments. In two experiments, we tested this prediction by asking participants to estimate the number of colored circles on a computer screen, with the the number of circles drawn from a color-specific distribution. When the distributions associated with each color overlapped, paricipants’ estimates were biased towards values intermediate between the two means, indicating that they grouped the stimuli into a single category. These data suggest that humans favor simpler explanations of sensory inputs. In contrast, when the distributions associated with each color overlapped minimally, the bias was reduced, indicating that sensory evidence for more complex explanations can override the simplicity bias.