Affordance

don norman

An affordance is something that provides the opportunity to perform an action. It is often described as a relationship between an object (or environment) and an organism. 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, but was later limited to only those an actor is aware of. The term has further evolved for use in the context of human–computer interaction (HCI) to indicate the easy discoverability of possible actions. The concept has application in several fields: perceptual, cognitive, and environmental psychology, industrial design, instructional design, science, technology and society (STS), and artificial intelligence.

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 his wife Eleanor, who created her theory of perceptual learning around this concept. Baltic German biologist Jakob von Uexküll made the first broad outlines of the concept in the early twentieth century, calling it the ‘functional tinting’ of organisms with respect to stimuli. A 2002 study 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.

In 1988, cognitive scientist and usability engineer Donald Norman appropriated the term 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 (the interdisciplinary study of systems in general) 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. Professor of design William Gaver divides affordances into three categories: perceptible (an affordance where information is available such that the actor perceives and can then act upon it), hidden (an affordance that indicates possibilities for action that are not noticed; e.g. it is not apparent from looking at a shoe that it could be used to open a wine bottle), and false (an apparent affordance that does not have any real function; e.g. placebo button). Perceptible affordances offer a direct link between perception and action. Hidden or false affordances can lead to mistakes and misunderstandings.

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