Asymmetry in the Müller-Lyer Illusion: Artifact or Genuine Effect?

The Müller-Lyer illusion was measured in thirty-five female subjects using the equation method. Three figures based on the illusion were used. One (outgoing fins) produced an illusion of expansion, another (ingoing fins) produced an illusion of contraction, and one was a plain line. The plain line was included to control for the error of the standard as a possible ‘contaminating factor’ which may produce asymmetry in the Müller-Lyer illusion. Although the error of the standard was found to correlate with the asymmetry to a statistically reliable degree, it accounted for only 39% of the size of the asymmetry and only 15% of the variance. At least some of the asymmetry is therefore attributable to the error of the standard, but the data indicate that there is another contributing factor (as yet unspecified). The possibility that the Müller-Lyer illusion may be inherently asymmetrical is discussed.