Visual Expert Human Factors: Product Misuse And "Affordances"

Marc Green

In many product liability cases, the defense will say that the plaintiff misused the product. Certainly, it is often easy to look back and to think that the victim behaved "stupidly." However, it should always be remembered the user's behavior must have seemed reasonable to him or her at the time - even if it doesn't seem so in hindsight. After all, the product user did not set out the purposely cause the accident. The question must then be asked:" Why did the behavior seem appropriate at the time?"

In product misuse, the answer often lies in the concept of perceptual "affordance." Briefly stated, an affordance is the property of an object that suggests its possible uses. For example, a person viewing a chair automatically sees that it affords sitting. So does a large rock and a railing. If your goal is to sit, then the sitting affordances offer a simple means for achieving the goal.

J. J. Gibson, the eminent perceptual psychologist, first suggested the notion 50 years ago. He said that when a person looks at an object, he directly perceives properties such as its shape, color, size, etc. Further, people perceive its affordances, the things that a person can do with it. Affordances are as much a perceptual property of an object as its color and shape. Gibson keenly noticed that perception and action are closely and automatically tied.

The key point in product liability is that these affordances may invite actions that the manufacturer had not intended or foreseen. The manufacturer will then say that the product was misused, but "misuse" is a matter of perspective. From the user perceptive, use is determined by affordances. Here are two examples where an affordance caused conflict between intended and unintended product use and an accident occurred:

A woman used a child's play table that appeared sturdy as a step to reach a high shelf. The table broke and the woman fell and was injured. The table was a low flat surface. It afforded stepping, so that is exactly what she did.

A driver hit and killed a child who was walking by the side of the road. The driver had put papers on the car's dashboard, which was a flat surface. The papers reflected up to the windshield and masked the driver's view of objects on the road. Clearly, the driver erred by obscuring his vision with placement of the papers. However, the flat surface of the dashboard was an affordance that invited placing objects on it, just as any desk or table would do.

Of course, affordances alone are seldom the entire story when an accident occurs. However, they can explain product misuse in some circumstances. The answer to the question, "Why did the behavior seem appropriate at the time?" is often that the product itself suggested a certain use, although it may not be one that the manufacturer had intended.

Of course, responses to affordances are not perfectly deterministic. Affordances invite an action but they do not force the action. Instruction, warnings and training may, in theory, prevent a person from acting on an affordance. For example, a "Wet Paint" sign in a railing may prevent a person from using the sitting affordance that the railing offers. Unfortunately, instruction, warnings and training are often unreliable ways to affect behavior. People are likely to fall back on primitive, natural tendencies, such as reliance on heuristics and affordances, when under stress or engaged in routine behavior.

Some people incorrectly talk about "learned affordances," which is an oxymoron. Affordances by their definition are unlearned. However, people may learn cultural norms that automatically guide behavior. One example of such a "population stereotype" is the adjusting of a knob to control magnitude. If a user wants to increase loudness on a new stereo, should he turn the "volume" knob clockwise or counterclockwise? Most people would automatically turn it clockwise because they have internalized that norm. Unfortunately, there is widespread confusion in some realms, such as computer interface design, between the concepts of affordance and population stereotype. In product design, however, the two concepts have similar implications - the designer must take both into account to create a proper design. A design that required a counterclockwise turn to increase volume would be faulty.

Affordances are only one example of the close tie between perception and action. Another example is "stimulus-response" compatibility, the natural connection between some percepts and some responses. I have discussed this elsewhere when describing the defective mapping between the hall lights in my house and the switches that control them.

Lastly, this discussion of affordances highlights a more general issue that frequently becomes the crux of litigation. What matters more, 1) the engineering/manufacturer perspective that sees the world in terms of intended function and preconceived notions about what is assumed true or 2) the cognitive/user perspective that is based perception, cognition, experience and what appears true?