Categorization with multiple items: Empirical and modeling results

Participants in categorization experiments usually assign a single stimulus to one of multiple categories. Despite the real-world significance, participants are rarely asked which of multiple options belong to a single category. In the current experiment, participants selected the stimulus, from a set of 2 or 3, that most likely to belongs to a learned category. The results of Experiment 1 (1-dimensional stimuli) suggest a repulsion effect, in which a nearby dominated stimulus reduced the probability of selecting the dominating stimulus. The results of Experiment 2 (2-dimensional stimuli) suggest a small attraction effect, in which the probability of selecting the dominating stimulus is increased. We extend standard exemplar-similarity models (GCM) by incorporating random utility modeling (RUM). The modeling results of both experiments suggest that stimulus utility alone may not be able to account for choice, i.e., the model must also incorporate similarity between choice options, although this finding is tentative for Experiment 2 and may represent a spatial bias.