Memory for category information is idealized through contrast with competing options.

We suggest that human category formation relies on contrastive learning mechanisms that seek to reduce prediction error. In keeping with this view, manipulating category contrast leads to systematic distortions in people's memory for category information. Simply by changing the basis of comparison (i.e., the available response options), we can systematically distort people's perceptions of novel, energy-source, and political categories. Our proposal explains perceived variations in category members' typicality, including cases in which average items are judged as highly typical and cases in which extreme or ideal members are judged as highly typical of the category. Although straightforward, our account spans findings from studies in goal-derived, cross-cultural, and object-based categorization and suggests ways in which society's perception of key issues is distorted by political discourse.