Source: Azjen, I. (1991). The theory of planned behaviour. Organisational Behaviour and Human Decision Processes, 50(2), 179–211. 

Abstract:  Research dealing with various aspects of the theory of planned behaviour (Ajzen, 1985, Ajzen, 1987) is reviewed, and some unresolved issues are discussed. In broad terms, the theory is found to be well supported by empirical evidence. Intentions to perform behaviours of different kinds can be predicted with high accuracy from attitudes toward the behaviour, subjective norms, and perceived behavioural control; and these intentions, together with perceptions of behavioural control, account for considerable variance in actual behaviour. Attitudes, subjective norms, and perceived behavioural control are shown to be related to appropriate sets of salient behavioural, normative, and control beliefs about the behaviour, but the exact nature of these relations is still uncertain. Expectancy-value formulations are found to be only partly successful in dealing with these relations. Optimal rescaling of expectancy and value measures is offered as a means of dealing with measurement limitations. Finally, inclusion of past behaviour in the prediction equation is shown to provide a means of testing the theory's sufficiency, another issue that remains unresolved. The limited available evidence concerning this question shows that the theory is predicting behaviour quite well in comparison to the ceiling imposed by behavioural reliability.

Insight:  In this seminal article, the author outlines how behaviour and behavioural intention to act in a certain way are affected by the attitude the subject has towards a particular behaviour, the subjective norm, and perceived behavioural control. The author developed our understanding of the fact that how humans intend to act may not be how they end up acting in a given situation. Intention, perception of behavioural control, attitude toward the behaviour, and subjective norm all reveal different aspects of the target behaviour and serve as possible directions for attack in attempts to alter particular behaviours, making this a powerful motivational theory in marketing. Read the paper to understand how the theory can be applied in practice to understand and predict individual behaviour in different circumstances.

URL:  https://www.sciencedirect.com/science/article/pii/074959789190020T