Extent of Illusory Effects for 32 Illusions

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Abstract
The presence of 32 familiar illusions among 39 Ss, about equally divided by sex, was moderate. Although several well-known illusions were found among a significantly high proportion of Ss (e.g., Mueller-Lyer), at least half of the illusions showed unexpected results: they varied widely in the number of Ss who reported them (e.g., Hering); and variations of the same basic type of illusion were as often effective as ineffective (e.g., Poggendorff). In most cases of divergent results among similar illusions, the stimulus differences were small. The findings did not vary between the sexes or between illusions of extent and duration. The diversity of the illusory response means that any one theory of illusions is not sufficient. Further research, rather than more theory, is required on the stimulus parameters of illusions.

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