The current study aimed to test the validity of an extended theory of planned behavior model (TPB; Ajzen, 1991), incorporating additional self and social influences, for predicting adolescent moderate-to-vigorous physical activity. Participants (N = 423) completed an initial questionnaire that assessed the standard TPB constructs of attitude, subjective norm, and perceived behavioral control, as well as past behavior, self-identity, and the additional social influence variables of group norms, family social support, friends’ social support, and social provisions. One week after completion of the main questionnaire, participants completed a follow-up questionnaire that assessed self-reported physical activity during the previous week. The standard TPB variables—past behavior, self-identity, and group norms, but not social support infuences—predicted intentions, with intention, past behavior, and self-identity predicting behavior. Overall, the results provide support for an extended version of the TPB incorporating self-identity and those social influences linked explicitly to membership of a behaviorally relevant reference group.
The authors are with the School of Psychology and Counselling, Queensland University of Technology, Brisbane, Queensland, Australia.