The Theory of Planned Behaviour (TPB) is a widely recognised framework for explaining and predicting human behaviour. Developed by Icek Ajzen, it proposes that behaviour is primarily guided by an individual’s intention to act, making intention the most immediate predictor of behaviour (Ajzen, 1991).
Intentions are shaped by three key components: attitude, subjective norms, and perceived behavioural control. Attitude reflects how positively or negatively a person evaluates a behaviour based on expected outcomes. Subjective norms refer to perceived social expectations and the influence of important others. Perceived behavioural control captures how capable a person feels and the extent to which they believe they have control over performing a behaviour, including external constraints such as time or resources. Together, these factors shape behavioural intention, which in turn guides behaviour. Perceived behavioural control can also directly influence behaviour when real-life barriers are present (Ajzen, 1991; Bartholomew Eldredge et al., 2016).
The TPB is valuable because it highlights the role of motivation in behaviour. This is illustrated in research on individuals with schizophrenia (Mausbach et al., 2013). The study found that subjective norms and perceived behavioural control were significant predictors of intention, while attitudes were not significant in the full model. In turn, intention was the strongest predictor of actual behaviour, such as engaging in daily tasks like socialising or managing routines. However, perceived behavioural control did not directly predict behaviour, despite being related at a simpler level of analysis. These findings suggest that individuals still make reasoned decisions about behaviour, but that not all components of the TPB operate equally strongly across contexts.
Despite its strengths, the model has some limitations. First, intentions do not always lead to behaviour, as factors such as time constraints, emotional states, unexpected events, or limited actual control can interfere with action (Ajzen, 2011). Second, not all behaviour is fully planned, as repeated actions can become habitual and are often triggered automatically by environmental cues rather than conscious intention (Ajzen, 2011).
Importantly, no single theory can fully explain human behaviour. Behaviour is complex and shaped by a wide range of influences, including habits, emotions, social context, and situational constraints. The TPB should therefore be understood as a useful framework that captures key aspects of decision-making, rather than a complete explanation.
Overall, the Theory of Planned Behaviour provides a structured way to understand how intentions, social influences, and perceived control contribute to behaviour, while also acknowledging that factors beyond the model influence real-world behaviour.
References:
Ajzen, I. (1991). The theory of planned behavior. Organizational Behavior and Human Decision Processes, 50(2), 179–211.
Ajzen, I. (2011). The theory of planned behaviour: Reactions and reflections. Psychology & Health, 26(9), 1113–1127. https://doi.org/10.1080/08870446.2011.613995
Bartholomew Eldredge, L. K., Markham, C. M., Ruiter, R. A. C., Fernández, M. E., Kok, G., & Parcel, G. S. (2016). Planning health promotion programs: An intervention mapping approach (4th ed.). Jossey-Bass.
Mausbach, B. T., Moore, R. C., Davine, T., Cardenas, V., Bowie, C. R., Ho, J., Jeste, D. V., & Patterson, T. L. (2013). The use of the theory of planned behavior to predict engagement in functional behaviors in schizophrenia. Psychiatry research, 205(1-2), 36–42. https://doi.org/10.1016/j.psychres.2012.09.016
