The TPB has been used to assess a variety of health-related behaviours. For example, Brubaker and Wickersham (1990) examined the role of the theory’s different components in predicting testicular self-examination and reported that attitude towards the behaviour, subjective norm and behavioural control (measured as self-efficacy) correlated with the intention to perform the behaviour. A further study evaluated the TPB in relation to weight loss (Schifter and Ajzen 1985). The results showed that weight loss was predicted by the components of the model; in particular, goal attainment (weight loss) was linked to perceived behavioural control. Similarly, Conner, Lawton et al. (2006) used the TPB to predict speeding behaviour using a driving simulator and an on-road speed camera and showed a significant role for most of the TPB variables in predicting both intentions and actual behaviour. Recently, O’Connor et al. (2006) also used the TPB to predict deliberate self-harm and suicidality at three months’ follow-up as a means to explore whether the TPB was relevant to more extreme behaviour and whether social cognitive variables were better predictors than clinical variables. The results showed a strong role for variables such as self-efficacy, attitude and descriptive norm and that these were better predictors than depression. There have now been several reviews and meta-analyses of the TPB which describe the extent to which this model can predict a range of health behaviours (Sheeran and Taylor 1999; Armitage and Conner 2001; Trafimow et al. 2002).

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.