The Theory of Planned Behavior (TPB) is a revision of an older social psychological theory of health behavior called the Theory of Reasoned Action (TRA). As the TPB really just builds upon the TRA, we will discuss them together and consider them as one overall theory.

TRA/TBP is an individual health behavior theory that focuses primarily on the relationship between people s attitudes and their behavior. The TRA/TBP presumes that attitudes coupled with subjective norms, an entity which it calls behavioral intentions precede and can predict behavior itself. Like the HBM, TRA/TPB focuses on rational, cognitive decision-making processes. It presumes that people are rational actors who use reason to make decisions about their actions (hence the name: Theory of Reasoned Action ).

The attitudes which spur health behaviors stem from assessments about the likely outcomes of such actions. If an individual strongly believes that some positive outcomes will very likely stem from a given action, that individual holds positive attitudes toward the action and we would anticipate that person to perform that action. Likewise, if an individual holds beliefs that a certain action is unlikely to produce a positive outcome or is likely to produce a negative outcome, that individual s negative attitudes would lead us to believe they are unlikely to perform that action.

These attitudes, coupled with subjective norms, help us to understand people s intentions about health behaviors. Subjective norms are personally-held beliefs about whether a given action will be approved or disapproved by the social groups that influence the individual, coupled with their motivation to conform to those norms. In plain English, do your family, friends, co-workers, and other influential others support the action you are thinking of taking, and do you care what they think?

Key constructs of the TRA

TRA states that an individual will perform some action X based on:

1. Their attitude towards performing the behavior.
2. The subjective norm associated with the behavior.

The TRA argues that there is a linear progression from behavioral and normative beliefs to behavioral intention and the behavior itself, so for example: A person believes that if she quits smoking she is likely to gain weight. If the peer social norm for females is to be as slim as possible and the behavior of quitting smoking itself is viewed as physically unpleasant, she may have a low intent to quit smoking. With a low intent to quit, she is unlikely to take any action toward quitting.

The TPB: A reformulation of the TRA

A major flaw of the TRA is that it does not account for external factors that may prevent someone from taking a specific action. The TRA presumes that we can predict behavior by understanding intention. Yet an individual can have strong attitudes about an action, and have a supportive social context, yet be unable to do the behavior. For example, an individual may hold a positive attitude about mammography and believe it will benefit her health: she may hold positive behavioral intentions to get a mammogram. She may also have a social network which is supports preventive health care measures generally, and mammography in particular. Yet if she cannot afford to receive a mammogram, or it is inconvenient for her to receive one, she may not be able to act on her behavioral intention.

To address this, a new element was added to TRA: perceived behavioral control (aka volitional control ). Perceived behavioral control refers to the degree to which a person believes they have control over whether they are able to take action. Perceived behavioral control is composed of two parts: control beliefs which concern the presence or absence of facilitators and barriers to performing the behavior, and perceived power of each of these factors to facilitate or inhibit the behavior. The reformulated TPB argues that a person s perception of how much they can control their behavior is combined with intention, attitude and norms to determine or predict behavior. Note the similarity of this concept with the concept of self-efficacy central to the HBM.

Components of the TRA/TBP

The TRA/TPB states that an individual s health behaviors are causally determined by:
1) His attitudes about the behavior, based upon his assessments about whether likely outcomes of the behavior are likely or not, positive or negative.
2) The subjective norms associated with the behavior: his beliefs about the opinions of members of his social group about the behavior as well as his desire to conform to those norms.
3) His behavioral intentions
4) His volitional control, which is composed of his beliefs about factors that will limit or enable his ability to act in accordance with his behavioral intentions and the amount of power he believes he has over performing the behavior.

Thus we may understand someone s decision to stop smoking using the TRA/TPB. An individual will stop smoking if: he holds positive attitudes about smoking cessation - that is, he believes that smoking is dangerous and strongly increases their risk of lung cancer, heart disease, and other negative health condition, and he believes that by quitting smoking he will dramatically reduce his risk of these conditions. Further, he will either need to be part of a social group that likewise thinks poorly of smoking, or, should his social group support smoking, the individual will need to decide that it is not important to him that they conform to the smoking norms of his social group. If he holds these attitudes and the relevant subjective norms are favorable, the individual will likely have a behavior intention to stop smoking. In order for him to actually act on this intention, he will need to believe that he is able to act on this desire - that he can afford a smoking cessation program - and that he is strong enough to avoid falling back into the habit of smoking.

Evidence and Critiques of the TRA/TPB

As discussed in the Montano and Kasprzyk article, this model has been used to understand a variety of different health behaviors with reasonable success. The first published test of the TRA/TPB (Schifter and Ajzen 1985) looked at precursors to weight loss. This study found that intention to lose weight could be accurately predicted based upon the attitudes, subjective norms, and volitional control reported by the individual. Yet volitional control and behavioral intentions were only moderately successful in predicting how much weight an individual would lose. Further, the researchers found evidence of an interaction between volitional control and behavioral intentions on weight loss - people were more likely to successfully lose weight if they had both strong behavioral intentions AND strong volitional control. This makes sense - wanting to lose weight is only part of the equation. The individuals most likely to be successful are those who both strongly want to lose weight and those who believe they are capable of doing so.

Another critique of the model is that the TPB construct volitional control is very unclear, as is its relationship to the actual control a person might have over his/her behavior. Some people lack self-confidence, and as such believe that many things are beyond their control, regardless of the reality of the situation. Likewise for particularly fatalistic individuals. How should these components of volitional control be accounted for? This is particularly a problem for those seeking to either utilize the theory in program design or seeking to evaluate the program.