The principle of aggregation, however, does not explain behavioural variability across situations, nor does it permit prediction of a specific behaviour in a given situation. It was meant to demonstrate that general attitudes and personality traits are implicated in human behaviour, but that their influence can be discerned only by looking at broad, aggregated, valid samples of behaviour. Their influence on specific actions in specific situations is greatly attenuated by the presence of other, more immediate factors.

Indeed, it may be argued that broad attitudes and personality traits have an impact on specific behaviours only indirectly by influencing some of the factors that are more closely linked to the behaviour in question.

Predicting Behaviour: Intentions and Perceived Behavioural Control

The theory of planned behaviour is an extension of the theory of reasoned action (Ajzen and Fishbein, 1980; Fishbein and Ajzen, 1975) made necessary by the original model's limitations in dealing with behaviours over which people have incomplete volitional control.

As in the original theory of reasoned action, a central factor in the theory of planned behaviour is the individual's intention to perform a given behaviour. Intentions are assumed to capture the motivational factors that influence a behaviour, they are indications of how hard people are willing to try, of how much of an effort they are planning to exert, in order to perform the behaviour. As a general rule, the stronger the intention to engage in a behaviour, the more likely should be its performance. It should be clear, however, that a behavioural intention can find expression in behaviour only if the behaviour in question is under volitional control, i.e., if the person can decide at will to perform or not perform the behaviour. Although some behaviours may in fact meet this requirement quite well, the performance of most depends at least to some degree on such non-motivational factors as availability of requisite opportunities and resources (e.g., time, money, skills, cooperation of others). Collectively, these factors represent people's actual control over the behaviour. To the extent that a person has the required opportunities and resources and intends to perform the behaviour, he or she should succeed in doing so.

The idea that behavioural achievement depends jointly on motivation (intention) and ability (behavioural control) is by no means new. It constitutes the basis for theorizing on such diverse issues as animal learning (Hull, 1943), level of aspiration (Lewin, Dembo, Festinger and Sears, 1944), performance on psychomotor and cognitive tasks (e.g., Pleishman, 1958; Locke, 1965; Vroom, 1964) and person perception and attribution (e.g., Heider, 1944; Anderson, 1974). It has similarly been suggested that some conception of behavioural control be included in our more general models of human behaviour, conceptions in the form of "facilitating factors" (Triandis, 1977), "the context of opportunity" (Sarver, 1983), "resources" (Liska, 1984), or "action control" (Kuhi, 1985). The assumption is usually made that motivation and ability interact in their effects on behavioural achievement. Thus, intentions would be expected to influence performance to the extent that the person has behaviour control and performance should increase with behavioural control to the extent that the person is motivated to try. Interestingly, despite its intuitive plausibility, the interaction hypothesis has received only limited empirical support.

Perceived behavioural control. The importance of actual behavioural control is self evident. The resources and opportunities available to a person must to some extent dictate the likelihood of behavioural achievement. Of greater psychological interest than actual control, however, is the perception of behavioural control and is impact on intentions and actions. Perceived behavioural control plays an important part in the theory of planned behaviour. In fact, the theory of planned behaviour differs from the theory of reasoned action in its addition of perceived behavioural control. Before considering the place of perceived behavioural control in the prediction of intentions and actions, it is instructive to compare this construct to other conceptions of control. Importantly, perceived behavioural control differs greatly from Rotter's (1966) concept of perceived locus of control. Consistent with an emphasis on factors that are directly linked to a particular behaviour, perceived behavioural control refers to people's perception of the ease or difficulty of performing the behaviour of interest. Whereas locus of control is a generalized expectancy that remains stable across situations and forms of action, perceived behavioural control can and usually does, vary across situations and actions. Thus, a person may believe that, in general, her outcomes are determined by her own behaviour (internal locus of control), yet at the same time she may also believe that her chances of becoming a commercial airplane pilot are very slim (low perceived behavioural control).

Another approach to perceived control can be found in Atkinson's (1964) theory of achievement motivation. An important factor in this theory is the expectancy of success, defined as the perceived probability of succeeding at a given task. Clearly, this view is quite similar to perceived behavioural control in that it refers to a specific behavioural context and not to a generalized predisposition. Somewhat paradoxically, the motive to achieve success is defined not as a motive to succeed at a given task but in terms of a general disposition "which the individual carries about him from one situation to another" (Atkinson, 1964, p. 242). This general achievement motivation was assumed to combine multiplicatively with the situational expectancy of success as well as with another situation-specific factor, the "incentive value" of success.

The present view of perceived behavioural control, however, is most compatible with Bandura's (1977, 1982) concept of perceived self-efficacy which "is concerned with judgements of how well one can execute courses of action required to deal with prospective situation" (Bandura, 1982, p. 122). Much of our knowledge about the role of perceived behavioural control comes from the systematic research program of Bandura and his associates (e.g., Bandura, Adams and Beyer, 1977; Bandura, Adams, Hardy and Howells, 1980). These investigations have shown that people's behaviour is strongly influenced by their confidence in their ability to perform it (i.e., by perceived behavioural control). Self-efficacy beliefs can influence choice of activities, preparation for an activity, effort expended during performance, as well as thought patterns and emotional reactions (see Bandura, 1982, 1991). The theory of planned behaviour places the construct of self-efficacy belief or perceived behavioural control within a more general framework of the relations among beliefs, attitudes, intentions and behaviour.

According to the theory of planned behaviour, perceived behavioural control, together with behavioural intention, can be used directly to predict behavioural achievement. At least two rationales can be offered for this hypothesis. First, holding intention constant, the effort expended to bring a course of behaviour to a successful conclusion is likely to increase with perceived behavioural control. For instance, even if two individuals have equally strong intentions to learn to ski and both try to do so, the person who is confident that he can master this activity is more likely to preserve than is the person who doubts his ability. The second reason for expecting a direct link between perceived behavioural control and behavioural achievement is that perceived behavioural control can often be used as a substitute for a measure of actual control. Whether a measure of perceived behavioural control can substitute for a measure of actual control depends, of course, on the accuracy of the perceptions. Perceived behavioural control may not be particularly realistic when a person has relatively little information about the behaviour, when requirements or available resources have changed, or when new and unfamiliar elements have entered into the situation. Under those conditions, a measure of perceived behavioural control may add little to accuracy of behavioural prediction. However, to the extent that perceived control is realistic, it can be used to predict the probability of a successful behavioural attempt (Ajzen, 1985).

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