Judgmental illusion produced by contrast with expectancy

Abstract

Judgments of the numerousness of dots vary inversely with the size of the background on which the dots are presented. An expectancy-contrast model is proposed and tested in an experiment with this size-numerosity illusion. According to the model, the expectancy for numerousness depends upon the subjective correlation between size and numerosity; judgments-of numerousness are assumed to reflect an additive contrast with the expectancy. In agreement with the model, the data demonstrate that the effects of size depend upon the correlation between size and numerosity; when this correlation is negative, the illusion is reversed. The model received tentative support from quantitative tests of fit, based on functional measurement procedures. Certain perceptual illusions have been attributed to the contrast between the judged stimulus and the expectancy produced by unjudged background cues (e.g., Brunswik & Herma, 1951). The size-weight illusion, in which the same weight seems lighter when presented in a larger package, is a familiar example. The expectancy interpretation assumes that everyday experience establishes a positive subjective correlation