Answer: The Muller-Lyer illusion is a trick of visual perception in psychology where two lines of the same length appear as if they are different lengths.
In psychology, the Muller-Lyer illusion is classically illustrated by showing subjects a pair of lines that are the same length. At the end of both of the lines are angled lines that come off of it. In one of the lines, the angled lines are acute to the main line, and for the other line they are obtuse to the main line. In general, when shown these two lines side by side, most people believe that the line with the obtuse angled “wings” is longer than the one with the acute angled “wings.”
The original test was designed by German sociologist Franz Carl Muller-Lyer in 1889. Although the illusion morphed into that described above, Muller-Lyer’s original test was done with an “arrow” figure, with one end of the line having obtuse wings and the other end having acute wings. Patients were then asked to mark the midpoint of that arrow. On average, they selected a midpoint that was closer to the “tail” end of the arrow, consistent with the perception that the obtuse wings elongate the length of the line while acute wings shorten the length.
Interestingly, there are major culture differences in the perception of line length in the Muller-Lyer illusion. These differences were demonstrated by the English anthropologist and neurologist W. H. R. Rivers in 1901. In his observation, he tested different cultures, such as a population of native aboriginal people called the Melanesians who live on Murray Island, a chain of volcanic islands just off Australia. In Rivers’ observations, these people were significantly less susceptible to the visual distortion effect of the changing angles of the wings. They more accurately identified the midpoint of the arrow, and were more likely to judge the two lines above as the same length compared to European patients. Doctor John Berry published a study in the British Journal of Psychology examining the Muller-Lyer response in a variety of populations, including the Inuit people from the Northern Arctic countries, some Scots from the city, and the Tenme people a West African people from Sierra Leone. A similar trend was observed.
One of the major studies regarding the perception of the Muller-Lyer illusion was published in 1966 (The influence of culture on visual perception). In this study, they looked at a total of seventeen different populations of people including European and American urban dwellers. In their experiment, they found that the urban Americans and European are more vulnerable to the line-length modifying wings of the Muller-Lyer illusion. In their explanation, the accuracy in perception for these people was due to their environment. They lived in a less rectilinear world compared to their European counterparts, whose right-angled buildings and streets made them more vulnerable to perceptual tricks such as the Muller-Lyer illusion. The study first used the word “carpentered,” which describes the type of squared atmosphere that the city-dwelling Americans and Europeans experience on a regular basis.
Explanation for the Muller-Lyer illusion
British neuropsychologist Richard Gregory first offered an explanation for the Muller-Lyer illusion in 1966. His explanation also relates to the carpentered, rectilinear environments that urban Europeans and Americans experience regularly. Consider a line with two acute angled wings are pointing towards the viewer. In Gregory’s explanation, these converging wings is interpreted by the perceptual systems of the brain that the line is closer to the viewer. On the other hand, a line that has obtuse angled wings points outwards towards the horizon, which diverges rather than converges.