Common factors versus uncommon variability in comparisons of the Poggendorff and Müller-Lyer illusions

Evidence for a common contributing factor in the Müller-Lyer and Poggendorff illusions is reviewed and complemented by correlational studies of the two illusions. However, these findings are compromised by consistent differences in the variability of the two illusory effects. In general, the variability associated with judgments of linear extent is consistently greater than the variability associated with alignment judgments. These findings were obtained with different figures, different stimulus parameters, and different response procedures. The difference in variability suggests problems for the notion of a common causal factor in illusions of linear extent and of direction. © 1993, Psychonomic Society, Inc.. All rights reserved