The term affordance was coined by JAMES JEROME GIBSON to describe the reciprocal relationship between an animal and its environment, and it subsequently became the central concept of his view of psychology, the ecological approach (Gibson 1979; Reed 1996; see ECOLOGICAL PSYCHOLOGY). An affordance is a resource or support that the environment offers an animal; the animal in turn must possess the capabilities to perceive it and to use it. "The affordances of the environment are what it offers animals, what it provides or furnishes, for good or ill" (Gibson 1977). Examples of affordances include surfaces that provide support, objects that can be manipulated, substances that can be eaten, climatic events that afford being frozen, like a blizzard, or being warmed, like a fire, and other animals that afford interactions of all kinds. The properties of these affordances must be specified in stimulus information. Even if an animal possesses the appropriate attributes and equipment, it may need to learn to detect the information and to perfect the activities that make the affordance useful -- or perilous if unheeded. An affordance, once detected, is meaningful and has value for the animal. It is nevertheless objective, inasmuch as it refers to physical properties of the animal's niche (environmental constraints) and to its bodily dimensions and capacities. An affordance thus exists, whether it is perceived or used or not. It may be detected and used without explicit awareness of doing so.
Affordances vary for diverse animals, depending on the animal's evolutionary niche and on the stage of its development. Surfaces and substances that afford use or are dangerous for humans may be irrelevant for a flying or swimming species, and substances that afford eating by an adult of the species may not be appropriate for a member in a larval stage. The reciprocal relationship between the environmental niche and a certain kind of animal has been dubbed the "animal-environment fit."
Utilization of an affordance implies a second reciprocal relationship between perception and action. Perception provides the information for action, and action generates consequences that inform perception. This information may be proprioceptive, letting the animal know how its body is performing; but information is also exteroceptive, reflecting the way the animal has changed the environmental context with respect to the affordance. Perceiving this relationship allows adaptive control of action and hence the possibility of controlling environmental change.
It is the functioning and description of the animal-environment encounter that is at the heart of research on affordances. Research has addressed three principal questions:
1. Do human adults actually perceive affordances in terms of task constraints and bodily requirements? The reality of perceptual detection of an animal-environment fit has been verified in experiments on adult humans passing through an aperture, reaching for objects with their own limbs or tools, judging appropriate stair heights for climbing, chair heights for sitting, and so forth. J. J. Gibson said that "to perceive the world is to coperceive oneself." In that case, actors should perceive their own body dimensions and powers in relation to the requirements of the relevant environmental resource or support. Warren and Whang (1987) investigated adults' judgments of aperture widths relative to their own body dimensions. Both wide- and narrow-shouldered adults rotated their shoulders when doorways were less than 1.3 times their own shoulder width. Scaling of the environment in terms of the natural yardstick of eye-height (Mark 1987; Warren 1984) has also been demonstrated.
2. Can stimulus information specifying
an affordance be described and measured? Can controlled actions
of an animal preparing for and acting on an affordance be observed
and measured? Gibson (1950) paved the way for this research by describing the
optic flow field created by one's own locomotion when flying
a plane. The specification by optical stimulus information of the
time for a moving animal to contact a surface or a target object
was described by Lee (1980), who showed that such information for
an observer approaching a surface could be expressed as a constant,
. The information is used in
controlling locomotion during braking and imminent collision by humans (Lee
1976) and by other animals (Lee and Reddish 1981; Lee, Reddish,
and Rand 1991). Effective information for heading (the direction
in which one is going) has been described by Warren (1995) in terms of
the global radial structure of the velocity field of a layout one
is moving toward.
Research on the action systems called into play and their controllability in utilizing an affordance has been the subject of study for reaching, standing upright, locomotion, steering, and so on. (Warren 1995). The control of reaching and grasping by infants presented with objects of diverse sizes shows accommodation of action to the object's size and shape by hand shaping, use of one or both arms, and so forth (see Bertenthal and Clifton 1997 for many details). Research of the specification of the affordance in stimulus information, and on control of action in realizing the affordance, converges in demonstrating that behavior is prospective (planned and intentional) and that stimulus information permits this anticipatory feature.
3. How do affordances develop cognitively and behaviorally? Developmental studies of affordances, especially during the first year, abound (Adolph, Eppler, and Gibson 1993). The behavior of crawling infants on a visual cliff (Gibson and Walk 1960) suggests that even infants perceive the affordances of a surface of support and avoid traversal of an apparent drop at an edge. Subsequent research has shown that duration of crawling experience is significantly related to dependable avoidance of the cliff, supporting other research demonstrating that LEARNING plays a role in detecting and responding effectively to many affordances, at least in humans. Development of action systems and increased postural control instigate the emergence of new affordance-related behavior. Babies begin to pay ATTENTION to objects and make exploratory reaches toward them as posture gradually enables reaching out and making contact with their surfaces (Eppler 1995).
As an infant learns about the constraints involved in the use of some affordance, learning may at first be relatively domain specific. Research by Adolph (1997) on traversal of sloping surfaces by crawling infants demonstrates that learning which slopes are traversable and strategies for successful traversal of them is not transferred automatically to traversal of the same slopes when the infant first begins upright locomotion. New learning is required to control the action system for walking and to assess the safety of the degree of slope. Learning about affordances is a kind of perceptual learning, entailing detection of both proprioceptive and exteroceptive information. The learning process involves exploratory activity, observation of consequences, and selection for an affordance fit and for economy of both specifying information and action.
The concept of affordance is central to a view of psychology that is neither mentalism nor stimulus-response BEHAVIORISM, focusing instead on how an animal interacts with its environment. Furthermore, the concept implies neither nativism nor empiricism. Rather, genetic constraints characteristic of any particular animal instigate exploratory activity that culminates in learning what its environment affords for it.
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