The current study aimed to test the validity of an extended theory of planned behavior model (TPB; Ajzen, 1991), incorporating additiol self and social influences, for predicting adolescent moderate-to-vigorous physical activity. Participants (N = 423) completed an initial questionire that assessed the standard TPB constructs of attitude, subjective norm, and perceived behavioral control, as well as past behavior, self-identity, and the additiol social influence variables of group norms, family social support, friends' social support, and social provisions. One week after completion of the main questionire, participants completed a follow-up questionire 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 influences-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.
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