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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 (R2 = 0.653; attitude, injunctive norms and perceived behavioural control significant) and behaviour (R2 = 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.
Conner et al. (1999) studied this question.