Interaction effects in the theory of planned behaviour: Studying cannabis use

Author(s): McMillan, B.

Journal/Book: Brit J Soc Psychol. 1999; 38: St Andrews House, 48 Princess Rd East, Leicester Le1 7Dr, Leics, England. British Psychological Soc. 195-222.

Abstract: This study employed the theory of planned behaviour (TPB) to investigate the factors underlying intentions and frequency of use of cannabis over a three-month period in a population of students (N = 249). In addition, several hypotheses in relation to the TPB were investigated. The TPB provided good predictions of both intentions (R-2 = 0.653; attitude, injunctive norms and perceived behavioural control significant) and behaviour (R-2 = 0.711; intentions significant). Other norm measures (descriptive and moral norms) explained additional variance in intentions (p < .01). In addition, habit strength and self-identity explained significant additional portions of the variance in intentions (P < .001), but not behaviour, over and above the TPB variables. Several interactions among these variables were also tested. Attitude moderated the impact of perceived behavioural control (PBC) on intentions (p < .001). Moral norms moderated the impact of attitudes on intentions (p < .001). Habit strength moderated the impact of self-identity on intentions (p < .001). PBC was found to moderate the impact of intentions on behaviour (p < .05). The findings are discussed in relation to how interaction effects further our understanding of the social processes by which variables are related in the TPB.

Note: Article Conner M, Univ Leeds, Sch Psychol, Leeds LS2 9JT, W Yorkshire, ENGLAND

Keyword(s): DRUG-USE; CONTINGENT CONSISTENCY; REASONED ACTION; EXERCISE BEHAVIOR; SOCIAL SUPPORT; FISHBEIN MODEL; SELF-EFFICACY; ROLE-IDENTITY; ATTITUDE; PREDICTION

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