7 PASCHAL SHEERAN,SARAH MILNE,THOMAS L. WEBB ANDPETER M. GOLLWITZER

IMPLEMENTATION INTENTIONSAND HEALTH BEHAVIOUR

1 General background

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 andimportant predictor of subsequent action, such as, the theory of plannedbehaviour (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 thatpeople give themselves to perform particular behaviours or to achievecertain goals (Triandis 1980) and are characteristicaIly measured by itemsof the form 'I intend to do/achieve X.' Intentions are the culmination of thedecision-making process; they signal the end of deliberation about abehaviour and capture the standard of performance that one has set oneself,one's commitment to the performance, and the amount of time and effortthat will be expended during action (GoIlwitzer 1990; Ajzen 1991; Webband Sheeran 2005). Given the centrality of the concept of intention tomodels of health behaviour, it is important to ask how weIl intentionspredict behaviour.

Sheeran (2002) approached this question by conducting a meta-analysisof meta-analyses of prospective tests of the intention-behaviour relation.Across 422 studies involving a sampie of 82,107 participants, intentionsaccounted for 28 per cent of the variance in behaviour, on average. R2 =0.28 constitutes a 'large' effect size according to Cohen's (1992) powerprimer, which suggests that intentions are 'good' predictors of behaviour.Moreover, 28 per cent of the variance may underestimate the 'true' relationbetween intention and behaviour because this value was not corrected for