Chapter 10 Implementation intentions

Several theories that have been used extensively to predict health behaviours construe a person’s intention to act as the most immediate and important predictor of subsequent action, such as the theory of planned behaviour (TPB; Ajzen 1991; Conner and Sparks, Chapter 5 this volume) and protection motivation theory (PMT; Rogers 1983; Norman et al., Chapter 3 this volume). Intentions can be defined as the instructions that people give themselves to perform particular behaviours or to achieve certain goals (Triandis 1980), and are characteristically measured by items of the form ‘I intend to do/achieve X’. Intentions are the culmination of the decision-making process; they signal the end of deliberation about a behaviour and capture the standard of performance that a person has set themselves, their commitment to the performance, and the amount of time and effort that will be expended during action (Gollwitzer 1990; Ajzen 1991). Given the centrality of the concept of intention to models of health behaviour, it is important to ask how well intentions predict behaviour. Sheeran (2002) approached this question by conducting a meta-analysis of meta-analyses of prospective tests of the intention–behaviour relation. Across 422 studies involving 82,107 participants, intentions accounted for an average of 28% of the variance in behaviour. This is a ‘large’ effect size according to Cohen’s (1992) power primer, and suggests that intentions are ‘good’ predictors of behaviour. However, Sheeran’s (2002) meta-analysis does not address whether changes in intentions predict changes in behaviour. To answer this question, Webb and Sheeran (2006) performed a meta-analysis of 47 experimental studies that demonstrated that a medium-to-large-sized change in intentions led to a small-to-medium-sized change in behaviour. This suggests that intentions do influence behaviour, but that intentional control of behaviour is more limited than previous meta-analyses of correlational studies have suggested. To investigate the sources of consistency and discrepancy between intention and behaviour, Orbell and Sheeran (1998) decomposed the intention–behaviour relation into a 2 (intention: