Vertical-Horizontal Illusion

The vertical-horizontal illusion is the tendency of observers to overestimate the length of a vertical line relative to a horizontal line of exactly the same length, an effect commonly demonstrated using a figure in which one line bisects another at a right angle; in the standard version of the illusion, the line that does the bisecting appears longer than the line that is bisected, even though the two lines are actually equal in length, and the illusion persists even for viewers who are already aware, and can verify, that both lines measure the same. Susceptibility to the illusion varies by cultural and physical environment: research has found that people from Western or urban settings tend to experience a stronger version of the illusion than people from eastern or open, rural landscape settings.