EBBINGHAUS ILLUSION: QUESTIONNING THE ROLE OF CONCEPTUAL SIMILARITY

The effect of social categorisation on optical illusions such as the Ebbinghaus illusion is an example of how social categorisation impinges on perceptual treatment of stimuli. The widespread explanation for this illusion relies on size contrast theory. Nevertheless, Stapel & Koomen's studies [28] have shown that increasing conceptual or social category similarity between central and surrounding elements enhances the illusion, suggesting that the perceptual processes involved occur at a later stage of processing. This effect diverges from a prediction based on Self Categorisation Theory, according to which increasing social category similarity decreases the illusion, since differences between elements belonging to the same category are minimised. We conceptually replicated Stapel & Koomen's study. Our results show an inverse pattern to theirs: while obtaining an illusion in all conditions, the illusion increased when category similarity decreased. Within a SCT perspective, we propose an explanation for the difference between these diverging results based on the social nature of the stimuli.