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The authors aimed to better understand why, and under what conditions, people take action to prevent, detect and diagnose disease [4], which they termed “health behaviours” (in contrast to “illness behaviours:” the behaviours a person who perceives themselves as ill may engage in to manage or treat their condition). In the case of tuberculosis screening, most people understood that the disease was serious (perceived seriousness), but many people did not believe they were likely to catch it (perceived susceptibility). Another factor which influenced the likelihood of undergoing screening for tuberculosis was the belief that screening was effective, or the benefit of early detection – the perceived benefit of engaging in the behaviour. This was weighed against the perceived barriers, such as fear of exposure to x-rays. All of these variables were likely influenced by modifying variables such as age and social norms. The authors observed that the act of finally deciding to engage in the health behaviour is prompted by a cue – an external event that causes the behaviour to change. This could be an event that changes the perceived threat (e.g. experiencing worrying symptoms or a friend developing tuberculosis) or a public health intervention, such as a screening campaign. Although originally intended to be a descriptive model, the Health Belief Model has also been applied both to design interventions and to predict health behaviours. The model construct "Perceived benefits and perceived barriers" has been shown to be the strongest predictor from this model [5]. A shortcoming of the Health Belief Model is its focus on individual choice, with no explicit mention of social influences or other external factors. It also assumes that health choices will be deliberate, thus ignoring unconscious choices (e.g. habit). The original model was formulated for relatively simple behaviours, such as getting a test or an inoculation. For more complex behaviours, perceived ability to perform the action (self-efficacy) is an issue. Self-efficacy was added to the model later, drawing from social cognitive theory [6]. 1.3. Theory of Reasoned Action, Theory of Planned Behaviour, and Reasoned Action Approach Shortly after publication of the Health Belief Model, Fishbein and Ajzen introduced the Theory of Reasoned Action (1967). While the Health Belief Model arose from the public health discipline, the Theory of Reasoned Action arose from social psychology Figure 1: The Health Belief Model [4] S.MedlockandJ.C.Wyatt /HealthBehaviourTheory inHealth Informatics148