Planned Behavior, Theory of

Why individuals choose to behave one way and not another has been a source of interest to scholars for decades. What role does a person's attitude play? Is social pressure a determining factor? Is past experience important? For obvious reasons, many science, health, and environmental communicators are interested in influencing behavior, whether this involves exercising more, saving energy, getting a mammogram, or planting a tree. If communicators knew what factors were most associated with a particular behavior, it might be possible to influence the outcome.

One of the most influential and well-supported social-psychological theories for predicting human behavior is called the theory of planned behavior (TPB). In hundreds of studies around the world, the theory has been successful in predicting behavior in a variety of contexts and with increasing specificity. Overall, the model has a good track record, with just three variables able to explain 30% to 50% of the variance in behavior.

The TPB model maintains that three independent variables—attitude, subjective norm, and perceived behavioral control—predict the dependent variable of behavior (or behavioral intention). The TPB is considered a deliberative processing model because it implies that people's beliefs and subsequent actions are formed after careful consideration of all the available information. Of course, behaviors are not always rational and thoroughly reasoned, which is a criticism of the model. Irrational decisions, habitual actions, or behaviors that are not consciously considered cannot be explained by this theory. Because the TPB treats behavior as solely under the control of the individual, it is most appropriately used with volitional or willful behavior and may poorly predict behaviors requiring skills or resources outside an individual's control.

The roots of the TPB are in the 1960s, when psychologist Martin Fishbein and others investigated the psychological processes by which attitudes “cause” behavior. In the 1970s, Fishbein and another psychologist, Icek Ajzen, developed the theory of reasoned action, a parsimonious model that held that a person's intent to behave in a certain way was largely a function of the person's attitude toward the act and social norms. The TPB extended the theory of reasoned action by incorporating the third independent variable, perceived behavioral control.

In designing a test of this theory, Ajzen stresses the “principle of compatibility,” which holds that each attitude and behavior has these elements of action: target (behavior), context, and time. It also maintains that the relationship between attitude and behavior will be greatest when both are measured at the same degree of specificity.

The theory has been employed to predict a variety of health-related behaviors, including exercise, smoking, diet, cancer screening, alcohol consumption, drug use, organ donation, and AIDS prevention. The model also has been used increasingly to predict various environmental behaviors, such as recycling, antinuclear activism, and water conservation. Another application of the TPB is predicting the dissemination and use of scientific research and willingness to spread science to the general public. Recently, the TPB has been used to predict behaviors in online environments and consumer behavior.