This diagram shows the famous Müller-Lyer illusion:  In 1889, Franz Carl Müller-Lyer, a German psychiatrist, published this optical illusion, which shows that the shape of the arrows tricks the brain into thinking that the top line is longer than the bottom. This illusion seemed universal until researchers in the 1950s and '60s found that people in many indigenous cultures around the world were immune to the illusion. This gave rise to the "carpenteered environments" hypothesis, which says that only people raised in environments with boxes and right angles are fooled by the illusion. This is because they perceive the lines as if they were edges of boxes, in which the line with inward facing arrows represents the far edge of a box, and therefore our brain adjusts for distance and thinks it is longer than it is. The hypothesis is still being debated today. This is an example of [[WEIRD psychology]].