The theory aims to predict an individual's intentions; whether they are to initiate or end the addictive behaviour, through measuring 3 things:
1. Attitudes Towards The Behaviour - these can either be positive or negative attitudes, as well as beliefs about the outcome.
2. Subjective Norms - The individual's perception of social norms and pressures to perform the behaviour, and whether or not they are able to comply to the pressures.
3. Perceived Behaviour Control - Whether or not the individual can carry out the behaviour when taking internal factors into consideration (skills, abilities and information) as well as external factors (obstacles and opportunities). This perceived control is then weighed up in the light of past behaviour.
(I have a fantastic acrostic for this but I'm not prepared to publicise it)
Support for this comes from Conner et al, who investigated the role of planned behaviour in smoking initiation in 11-12 year olds by testing 675 adolescents through various measures, including TPB, and 9 months later checking to see if any of them took up smoking. It was found that behavioural intentions were generally a good predictor for later smoking behaviour.
Furthermore, research from Guo, who tested 14 thousand Chinese adolescents, also found that TPB to be a useful predictor for later smoking behaviour.
Both studies highlight the theory to be very effective, even in non-western cultures.
Overall, TPB has proven to be a much suited and influential model, which is currently the most popular and widely used social condition model in health psychology. It also takes a variety of variables into account, as it considers past behaviour and addresses social and environmental factors.
TPB also has substantial empirical support, and has proven very successful with smoking behaviours. However, it does not take into account them emotional disappointment that may occur if the planned behaviour does not work for an individual.