It is a common frustration for market researchers and brand managers alike that what consumers say is often different to what they actually do. Research might show that consumers have a positive attitude towards an issue, but the actual behaviour shows otherwise.
In classic consumer behaviour theory, we are familiar with the trichotomy of cognition-affection-action that becomes the basis for the brand funnel concept. Theoretically, consumers who are aware of the brand and have a positive attitude towards the brand are likely to take the final action: purchase. A smoker might be aware and afraid that smoking is not good for his health, but that does not necessarily stop him from smoking. This is where the Attitude-Behaviour Discrepancy happens, which shows that the correlation between attitude and behaviour in many cases is very weak or event nonexistent.
However, that does not necessarily means that attitude cannot be used to predict behaviour. This discrepancy can be explained without having to disregard attitude as a factor or without having to conclude that attitude is not correlated with behaviour.
In connecting attitude and behaviour, most of the time we fail to measure attitude correctly because we only use one measurement variable and at one point of time only. Moreover, the resulting behaviour may be caused not by one attitude but the result of an interaction of various - sometimes conflicting - attitudes. Studies using more than one relevant attitude to predict behaviour have generally produced stronger results. In the smoker's case, the subject might have a cognition that a healthy lifestyle is very hard to achieve, or he believes that a heart attack is not something that might happen to him.
The third explanation why attitudes do not correlate with behaviour is because the situational context is different. Herbert W. Simons wrote that behaviour is a joint function of attitude and situational influences, or as Rokeach put it: attitude towards objects and attitudes towards situations. Understanding the driver behind a behaviour requires understanding not just consumers' attitude towards the object, but also the situational influences that drive them to act in a certain behaviour. Therefore, as a market researcher, it is important to always explore conflicting attitudes and situations that drive the behaviour.