The importance of being nonalignable: a critical test of the structural alignment theory of similarity.

The structural alignment theory of similarity distinguishes 2 types of difference that may occur between stimuli: Alignable differences are those related to a commonality, whereas nonalignable differences are not related to a commonality. Alignment theory predicts that alignable differences should be more heavily weighted than nonalignable differences in similarity judgment. Experiments 1 and 2 demonstrate that, contrary to this prediction, nonalignable differences exerted a greater impact than alignable differences in similarity and difference judgments of geometric stimuli. Experiment 3 revealed that the relative weight accorded a given difference was also affected by contextual constraints. Thus, although the experiments supported the validity of the distinction between alignable and nonalignable differences, results were discordant with the specific prediction of structural alignment theory.