Shared and distinct individual-difference structures across geometrical illusions

Abstract Principles proposed to explain geometrical illusions have often been assumed to apply across multiple illusions. However, it remains unclear whether these illusions rely on shared perceptual mechanisms. In the present study, we focused on five geometrical illusions linked to spatial perception and examined the structure of individual differences in illusion magnitude. All illusions were measured repeatedly using the method of adjustment with balanced response series to improve measurement reliability. In addition, illusion-specific control conditions were used to derive residualized illusion magnitudes, allowing us to examine inter-illusion correlations after controlling for individual differences attributable to the adjustment task in general. The results revealed moderate positive correlations among the Railroad (size-constancy) illusion, Ponzo illusion, and Poggendorff illusion. This pattern was supported by complementary exploratory analyses, including factor analysis, hierarchical clustering, and multidimensional scaling. By contrast, the Kanizsa shrinkage illusion appeared distinct from the others, and the Müller–Lyer illusion showed little relation to this structure despite its historical association with spatial perception. These findings suggest that individual-difference structures can reveal shared components among specific geometrical illusions and help constrain theories of visual illusions.