Schwarzer (1992) has criticized the TPB for its omission of a temporal element and argues that the TPB does not describe either the order of the different beliefs or any direction of causality. However, in contrast to the HBM and the PMT, the model attempts to address the problem of social and environmental factors (in the form of normative beliefs). In addition, it includes a role for past behaviour within the measure of perceived behavioural control.

If applied to alcohol consumption, the TPB would make the following predictions: if an individual believed that reducing their alcohol intake would make their life more productive and be beneficial to their health (attitude to the behaviour) and believed that the important people in their life wanted them to cut down (subjective norm), and in addition believed that they were capable of drinking less alcohol due to their past behaviour and evaluation of internal and external control factors (high behavioural control), then this would predict high intentions to reduce alcohol intake (behavioural intentions). The model also predicts that perceived behavioural control can predict behaviour without the influence of intentions. For example, if perceived behavioural control reflects actual control, a belief that the individual would not be able to exercise because they are physically incapable of exercising would be a better predictor of their exercising behaviour than their high intentions to exercise.

The TPB emphasizes behavioural intentions as the outcome of a combination of several beliefs.
The theory proposes that intentions should be conceptualized as ‘plans of action in pursuit of behavioural goals’ (Ajzen and Madden 1986) and are a result of the following beliefs:
■ Attitude towards a behaviour, which is composed of either a positive or negative evaluation of a particular behaviour and beliefs about the outcome of the behaviour (e.g. ‘exercising is fun and will improve my health’).
■ Subjective norm, which is composed of the perception of social norms and pressures to perform a behaviour and an evaluation of whether the individual is motivated to comply with this pressure (e.g. ‘people who are important to me will approve if I lose weight and I want their approval’).
■ Perceived behavioural control, which is composed of a belief that the individual can carry out a particular behaviour based upon a consideration of internal control factors (e.g. skills, abilities, information) and external control factors (e.g. obstacles, opportunities), both of which relate to past behaviour.
According to the TPB, these three factors predict behavioural intentions, which are then linked to behaviour. The TPB also states that perceived behavioural control can have a direct effect on behaviour without the mediating effect of behavioural intentions.

The theory of reasoned action (TRA) was extensively used to examine predictors of behaviours and was central to the debate within social psychology concerning the relationship between attitudes and behaviour (Fishbein 1967; Ajzen and Fishbein 1970; Fishbein and Ajzen 1975). The theory of reasoned action emphasized a central role for social cognitions in the form of subjective norms (the individual’s beliefs about their social world) and included both beliefs and evaluations of these beliefs (both factors constituting the individual’s attitudes). The TRA was therefore an important model as it placed the individual within the social context and in addition suggested a role for value which was in contrast to the traditional more rational approach to behaviour. The theory of planned behaviour (TPB) was developed by Ajzen and colleagues (Ajzen 1985; Ajzen and Madden 1986; Ajzen 1988) and represented a progression from the TRA.

Social cognition models examine factors that predict behaviour and/or behavioural intentions and in addition examine why individuals fail to maintain a behaviour to which they are committed. Social cognition theory was developed by Bandura (1977, 1986) and suggests that behaviour is governed by expectancies, incentives and social cognitions. Expectancies include:
■ situation outcome expectancies: the expectancy that a behaviour may be dangerous (e.g. ‘smoking can cause lung cancer’)
■ outcome expectancies: the expectancy that a behaviour can reduce the harm to health (e.g. ‘stopping smoking can reduce the chances of lung cancer’)
■ self-efficacy expectancies: the expectancy that the individual is capable of carrying out the desired behaviour (e.g. ‘I can stop smoking if I want to’).
The concept of incentives suggests that a behaviour is governed by its consequences. For example, smoking behaviour may be reinforced by the experience of reduced anxiety, having a cervical smear may be reinforced by a feeling of reassurance after a negative result.
Social cognitions are a central component of social cognition models. Although (as with cog- nition models) social cognition models regard individuals as information processors, there is an important difference between cognition models and social cognition models – social cognition models include measures of the individual’s representations of their social world. Accordingly, social cognition models attempt to place the individual within the context both of other people and the broader social world. This is measured in terms of their normative beliefs (e.g. ‘people who are important to me want me to stop smoking’).

The PMT has been less widely criticized than the HBM; however, many of the criticisms of the HBM also relate to the PMT. For example, the PMT assumes that individuals are conscious information processors; it does not account for habitual behaviours, nor does it include a role for social and environmental factors.

Rippetoe and Rogers (1987) gave women information about breast cancer and examined the effect of this information on the components of the PMT and their relationship to the women’s intentions to practise BSE. The results showed that the best predictors of intentions to practise BSE were response effectiveness, severity and self-efficacy. In a further study, the effects of persuasive appeals for increasing exercise on intentions to exercise were evaluated using the components of the PMT. The results showed that susceptibility and self-efficacy predicted exercise intentions but that none of the variables was related to self-reports of actual behaviour. In another study, Beck and Lund (1981) manipulated dental students’ beliefs about tooth decay using persuasive communication. The results showed that the information increased fear, and that severity and self-efficacy were related to behavioural intentions. Norman et al. (2003) also used the PMT to predict children’s adherence to wearing an eye patch. Parents of children diagnosed with eye problems completed a baseline questionnaire concerning their beliefs and a follow-up questionnaire after two months describing the child’s level of adherence. The results showed that perceived susceptibility and response costs were significant predictors of adherence.

If applied to dietary change, the PMT would make the following predictions: information about the role of a high fat diet in coronary heart disease would increase fear, increase the individual’s perception of how serious coronary heart disease was (perceived severity), and increase their belief that they were likely to have a heart attack (perceived susceptibility/susceptibility). If the individual also felt confident that they could change their diet (self-efficacy) and that this change would have beneficial consequences (response effectiveness), they would reporthigh intentions to change their behaviour (behavioural intentions). This would be seenas an adaptive coping response to the information.