Explanations
The Müller-Lyer illusion (1889): two lines of equal length appear different because of their endings.
Depth hypothesis (Gregory 1963): arrowheads mimic corners — outward fins look like an inside corner (far), inward like an outside corner (near). The brain scales perceived length by assumed depth.
Conflation hypothesis (Pressey): wings bias the centroid, shifting the perceived endpoint location.
Statistical theory (Howe & Purves 2005): lines ending in outward fins more often subtend larger physical lengths in natural scenes — Bayesian prior causes overestimation.
Bias is ~20% on average and persists even when you know it's an illusion.