This is part of the Human perception projectBack
Although it has long been apparent that observers tend to overestimate the magnitude of acute angles and underestimate obtuse ones, there is no consensus about why such distortions are seen. Geometrical modeling combined with psychophysical testing of human subjects indicates that these misperceptions are the result of an empirical strategy that resolves the inherent ambiguity of angular stimuli by generating percepts of the past significance of the stimulus rather than the geometry of its retinal projection.
Why are angles misperceived?
Nundy, S., Lotto, R.B., Coppola, D, Shimpi, A. and Purves, D. (2000)
Proceedings of the National Academy of Science USA 97:5592-5597.