Affordances

An "affordance" (introduced in J.J. Gibson's theory of ecological psychology [1, 2]) is a relationship between an actor (i.e., person or animal) and an object: specifically, it refers to the possibility of the actor performing some action with the object, whether or not the actor can perceive that possibility (see this synopsis of Gibson's definition of the term for a more involved explanation). For instance, the typical school desk affords sitting for (most) people. It also affords moving, throwing, sleeping, and disposing of chewed-up gum (though many may be unaware of it--a "hidden affordance"). To another actor, say a banana slug, the same desk will have different affordances (climbing, perhaps).

A "perceived affordance" is one the actor perceives as possible, whether or not that is actually the case. This is what Don Norman originally called an affordance in [4]; his observation was that "the appearance of the device could provide the critical clues required for its proper operation" (see [5] for his retraction of the initial misuse of the term, and more useful discussion). Perceived affordances are developed (in contrast to real affordances, which exist naturally) from a combination of what one sees and what one knows (i.e., one's conceptual model). A door with a pull bar provides a simple example of a perceived affordance. Using one's knowledge of doors and pull bars, one may perceive that the door affords opening by pulling. If it actually does not (a "false affordance"), confusion (or worse) will likely arise.

Consequently, perceived affordances are extremely relevant to user interface design (UID). They are closely linked to the notion of discoverability: the degree to which users can figure out what it is possible to do with an interface (and how to do it). It is therefore important, when designing a user interface, to map real affordances to perceived affordances as much as possible (it is sometimes useful to violate this principle, with cheats in computer games for instance [6]). For example, the content your webpage seems to offer should be there, and the way the navigation scheme seems to work should be the way it works. These mappings can be created using image schemas [3] (e.g., up is better), and social values (e.g., green means go, red means stop).

References

[1] 

Gibson, J.J. "The Theory of Affordances." Perceiving, Acting and Knowing: Toward an Ecological Psychology. Eds. R.E. Shaw, and J. Bransford. Hillsdale, NJ: Lawrence Erlbaum Associates, 1977. 67-82.

[2] 

Gibson, J.J. The Ecological Approach to Visual Perception. Boston: Houghton Mifflin, 1979.

[3] 

Lakoff, George, and Mark Johnson. Metaphors We Live By. Chicago: University of Chicago Press, 1980.

[4] 

Norman, Donald A. The Psychology of Everyday Things. New York: Basic Books, 1988.

[5] 

Norman, Donald A. "Affordance, Conventions, and Design." Interactions May 1999: 38-43.

[6] 

Norman, Donald A. "Affordance and Design." Online posting. jnd.org. 15 May 2003 <http://www.jnd.org/dn.mss/affordances-and-design.html>.


©2003 Dana Dahlstrom and Vinu Somayaji. All rights reserved.
Last modified: Wed, 04 Jun 2003 23:37:59 -0700.