Outline how psychologists would explain the Müller-Lyer illusion.

Richard Gregory's constructivist theory of perception provides the most common explanation for the Müller-Lyer illusion. He argued that perception is an active process of 'hypothesis testing' based on past knowledge and experience. In a world filled with buildings (a 'carpentered world'), we learn to interpret angles as depth cues. The fins-in figure looks like the far corner of a room, while the fins-out figure looks like the near corner of a building. Our brain uses this depth information and applies size constancy scaling. If two objects cast the same size image on our retina, but one is perceived as being further away, our brain concludes the further object must be larger. This is 'misapplied' constancy because the depth cues are on a 2D surface, leading to the illusion.