7 Paschal Sheeran , Sarah Milne , Thomas L . Webb and Peter M . Gollwitzer Implementation Intentions and Health Behaviour

1. 1 The intention-behaviour relation Several theories that have been used extensively to predict health behaviours construe the 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 in this volume) and protection motivation theory (PMT; Rogers 1983; Norman et al., Chapter 3 in 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 characteristicaIly 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 one has set oneself, one's commitment to the performance, and the amount of time and effort that will be expended during action (GoIlwitzer 1990; Ajzen 1991; Webb and Sheeran 2005). Given the centrality of the concept of intention to models of health behaviour, it is important to ask how weIl 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 a sampie of 82,107 participants, intentions accounted for 28 per cent of the variance in behaviour, on average. R 2 = 0.28 constitutes a 'large' effect size according to Cohen's (1992) power primer, which suggests that intentions are 'good' predictors of behaviour. Moreover, 28 per cent of the variance may underestimate the 'true' relation between intention and behaviour because this value was not corrected for