MüllerLyer

The Müller-Lyer illusion, also called the Müller-Lyer illusion, is a classic optical illusion in which two horizontal lines of identical physical length are perceived as different lengths because of the shape of arrow-like fins at their ends. When the fins point outward, the line tends to be perceived as longer than when the fins point inward, even though both lines are the same length. The effect is robust across observers and can be demonstrated with simple drawings, photographs, and even three-dimensional cues.

The illusion was first described by the German psychologist Franz Carl Müller-Lyer in 1889. It has since been studied extensively as a demonstration of how context and depth cues influence size perception, challenging simple notions of veridical vision and highlighting the role of cognitive processing in early visual interpretation.

Proposed explanations fall mainly into depth- and context-based accounts. A common idea is that the fins cue depth orientation, leading the visual system to interpret one line as being farther away and therefore longer to produce a consistent perceived size. The carpentered-world hypothesis suggests that exposure to rectilinear environments with corners and right angles biases people to interpret line endings as depth cues, increasing susceptibility to the illusion. Cross-cultural research shows variability in the illusion's strength, typically stronger in urban, industrialized populations and weaker in societies with fewer depth cues in the environment, though effects can be modulated by viewing conditions and task demands.

The Müller-Lyer illusion remains a foundational example in perception research, illustrating size constancy, context effects, and the interaction between bottom-up sensory information and top-down interpretation.