Theory of Planned Behaviour

Ajzen (1985, 1991) proposed the theory of planned behaviour to explain behaviour under our control (deliberate behaviours). This theory is centred around a person's intention. Ajzen states that our behaviour can be predicted from our intention to behave in a certain way.

Applied to addiction, TPB explains drug abstinence by referring to addicts' intention to give up drugs. Such intention arises from three key influences: our personal attitudes, beliefs about what others think, and beliefs about our ability to change our behaviour.

Personal attitudes in the theory of planned behaviour

This refers to the addict’s attitude towards their addiction, such as if they think their addiction is good or bad. To work this out, they weigh up the pros and cons of their addiction and often, if the good outweighs the bad, they will continue their addiction.

Subjective norms in the theory of planned behaviour

This refers to the individual’s beliefs about what their close friends and family think of their addiction and is influenced by norms of behaviour. The addicted person considers how their friends and family respond to their addiction. If they think they would view it unfavourably, they are less likely to engage in their addiction, i.e. their intent changes.

Perceived behavioural control in the theory of planned behaviour

Perceived behaviour control refers to how much a person believes they have control over their behaviour, also known as ‘self-efficacy’. This depends on a person’s perception of their willpower, ability and resources that could enable them to give up their addiction.

According to TPB, perceived behaviour control has two potential effects:

• The more control you think you have, the stronger your intention to give up your addiction.
• The greater your perceived control over your behaviour, the longer and harder you will try to stop.

Representation of thought bubbles describing TPB, Canva.

Evaluation: strengths and weaknesses of the theory of planned behaviour

Let’s study some of the weaknesses and strengths of the theory of planned behaviour.

Research support (TPB)

Hagger et al. (2011) tested TPB for alcohol-related addictions. They found that personal attitudes, subjective norms and perceived behavioural control affected an addict’s intention to quit their addiction. Both intention and perceived behavioural control also influenced the number of units an addict consumed after one and three months. This intention led to attempts to give up addictive behaviour.

Although this research supports TPB, the period between intention and behaviour was short. Also, the theory doesn’t predict behaviour for all addictions, such as binge drinking, meaning that TPB’s success might depend on the addiction studied.

Limited explanation

TPB is based on logic, reasoning, decision making and choice. This means it has difficulty accounting for less rational factors that influence addiction intentions and behaviours, such as emotions and cognitive bias.

Methodological problems

Much research into TPB uses self-report techniques and is therefore liable to social desirability bias. This is because the emphasis on perception means that researchers have to ask addicts about their attitudes and perceptions. However, this means that people may try to ‘look good’ and not be truthful due to social desirability bias.

Another methodological problem with research into TPB is that they are often correlational, which means we cannot establish a causal relationship between TPB and addiction behaviour.

TPB research depends on self-report techniques, Canva.

Short term versus long term effectiveness

McEchan et al. (2011) found that intention only predicted that an addict would give up their addiction if the space between intention and giving up was small.

They conducted a meta-analysis of 237 health behaviour studies (addiction-related) and their links to TPB. They found that the strength of the correlation between intentions and behaviour varied according to the length of time between the two; the smaller the period in between, the stronger the intention. For example, with drinking, this period should be about five weeks.

This means that intention is only a valid explanation for giving up addiction in the short term. This may explain why the research evidence on TPB is so mixed, meaning it is not an entirely valid explanation of drug-related behaviour.

Addiction behaviour

A limitation of TPB is that although it explains intention, there is a gap between intention and behaviour. For example, Miller and Howell (2005) studied gambling in underage teens and found a strong correlation between attitudes, norms, perceived control and intention but no link between this intention and behaviour.

Many psychologists now question whether TPB is a model of behavioural change at all. It focuses on intention, and it is challenging to create drug-related interventions that could connect this intention and behaviour.