Carpenteredness

The Müller-Lyer illusion. The lines ending in arrows seem shorter than those ending in a "tail" though all lines are of the same length.

Carpenteredness is an early theory in the psychology of perception, to explain cultural variations in the susceptibility to illusions such as the Müller-Lyer illusion, where a line seems smaller if it has arrows on both sides, as opposed to a tail.

History

Around the turn of the century, W. H. R. Rivers had noted that natives of the Australian Murray Island were less susceptible to the Muller-Lyer illusion. Rivers suggested that this difference may be due to Europeans living in more rectilinear environments. Similar results were also observed by John Berry in his work on Eskimo groups (1968, 1971). .

In 1965, following a debate between Donald T. Campbell and Melville J. Herskovits on whether culture can influence such basic aspects of perception such as the length of a line, they suggested that their student Marshall Segall investigate the problem. In their definitive paper of 1966, they investigated seventeen cultures and showed that people in different cultures differ substantially on how they experience the Müller-Lyer stimuli. They write

European and American city dwellers have a much higher percentage of rectangularity in their environments than non-europeans and so are more susceptible to that illusion.

They also used the word "carpentered" for the environments that Europeans mostly live in - characterized by straight lines, right angles, and square corners.

These conclusions were challenged in later work by Gustav Jahoda, who tested members of an African tribe living in a traditional rural environment vs. members of same group living in African cities. Here, no significant difference in susceptibility to the M-L illusion was found. Subsequent work by Jahoda suggested that retinal pigmentation may have a role in the differing perceptions on this illusion, and this was verified later by Pollack (1970). It is believed now that not "carpenteredness", but the density of pigmentation in the eye is related to susceptibility to the M-L illusion. Dark-skinned people often have denser eye pigmentation.

Thus carpenteredness - the degree to which an environment has straight lines and right angles, may not be the primary reason for human variations on the M-L illusion.