Affordance

"Afford" redirects here. For the surname, see Afford (surname).

The handles on a tea set provide an obvious affordance for holding.

An affordance is a relation between an object or an environment and an organism that, through a collection of stimuli, affords the opportunity for that organism to perform an action. For example, a knob affords twisting, and perhaps pushing, while a cord affords pulling. As a relation, an affordance exhibits the possibility of some action, and is not a property of either an organism or its environment alone.

Different definitions of the term have developed. The original definition described all actions that are physically possible. This was later adapted to describe action possibilities of which an actor is aware. Some define affordance as a potential resource for some (not a particular) organism or species of organism, and so while inviting the possible engagement of some species, not identified with any particular one. The term has further evolved for use in the context of human–computer interaction (HCI) to indicate the easy discoverability of possible actions.

The word is used in a variety of fields: perceptual psychology, cognitive psychology, environmental psychology, industrial design, human–computer interaction, interaction design, communication studies, instructional design, science, technology and society (STS), and artificial intelligence.

As action possibilities

Psychologist James J. Gibson originally introduced the term in his 1977 article "The Theory of Affordances" and explored it more fully in his book The Ecological Approach to Visual Perception in 1979. He defined affordances as all "action possibilities" latent in the environment, objectively measurable and independent of the individual's ability to recognize them, but always in relation to agents and therefore dependent on their capabilities. For instance, a set of steps which rises four feet high does not afford the act of climbing if the actor is a crawling infant. Gibson's is the prevalent definition in cognitive psychology.

Affordances were further studied by Eleanor J. Gibson, wife of James Gibson, who created her theory of perceptual learning around this concept. Eleanor Gibson's book, An Ecological Approach to Perceptual Learning and Development explores affordances further.

Jakob von Uexküll had already discussed the concept in the early twentieth century, calling it the "functional tinting" (funktionale Tönung) of organisms with respect to stimuli.

Anderson, Yamagishi and Karavia (2002) sought to determine whether visual attention or affordance forms the basis of the motor signals generated by many everyday graspable objects. By examining how the properties of an object affect an observer’s reaction time for judging its orientation, they provided evidence to indicate that directed visual attention (not affordance) is responsible for the automatic generation of many motor signals associated with the spatial characteristics of perceived objects.

As perceived action possibilities

In 1988, Donald Norman appropriated the term affordances in the context of human–machine interaction to refer to just those action possibilities that are readily perceivable by an actor. Through his book The Design of Everyday Things, this interpretation was popularized within the fields of HCI and interaction design. It makes the concept dependent not only on the physical capabilities of an actor, but also the actor's goals, plans, values, beliefs, and past experiences. If an actor steps into a room with an armchair and a softball, Gibson's original definition of affordances allows that the actor may throw the chair and sit on the ball, because this is objectively possible. Norman's definition of (perceived) affordances captures the likelihood that the actor will sit on the armchair and throw the softball. Effectively, Norman's affordances "suggest" how an object may be interacted with. For example, the size and shape of a softball obviously fit nicely in the average human hand, and its density and texture make it perfect for throwing. The user may also bring past experiences to bear with similar objects (baseballs, perhaps) when evaluating a new affordance.

Norman's 1988 definition makes the concept of affordance relational rather than subjective or intrinsic. This he deemed an "ecological approach", which is related to systems-theoretic approaches in the natural and social sciences. The focus on perceived affordances is much more pertinent to practical design problems from a human-factors approach, which may explain its widespread adoption.

Norman later explained that this restriction in meaning of the term had been unintended, and that he would replace the term by "perceived affordance" in any future revision of the book. However, the definition from his book has been widely adopted in HCI and interaction design, and both meanings are now commonly used in these fields.

The different interpretations of affordances, although closely related, can be a source of confusion in writing and conversation if the intended meaning is not made explicit and if the word is not used consistently. Even authoritative textbooks can be inconsistent in their use of the term.

False affordances

William Gaver divided affordances into three categories: perceptible, hidden, and false.

• A false affordance is an apparent affordance that does not have any real function, meaning that the actor perceives nonexistent possibilities for action. A good example of a false affordance is a placebo button.
• A hidden affordance indicates that there are possibilities for action, but these are not perceived by the actor. For example, it is not apparent from looking at a shoe that it could be used to open a wine bottle.
• For an affordance to be perceptible, there is information available such that the actor perceives and can then act upon the existing affordance.

This means that, when affordances are perceptible, they offer a direct link between perception and action, and, when affordances are hidden or false, they can lead to mistakes and misunderstandings.

See also

• Usability
• Action-specific perception
• Form follows function
• Ambient optic array

Additional reading

• The role of affordances in evolution and niche construction is discussed by
• The role of affordances in agency is discussed by
• Affordance and computation is discussed by

External links

• A series of slides concerning theories of vision and (incidentally) the role of affordances and some interesting optical illusions concerning affordances