The presence of two subcomponents within each theory of planned behaviour (TPB) concept of attitude (affective and instrumental), subjective norm (injunctive and descriptive), and PBC (self-efficacy and controllability) has been widely supported. However, research has not examined whether the commonality of variance between these components (i.e. a general factor) or the specificity of variance within the subcomponents influences intention and behaviour. Therefore, the purpose of this study was to examine the optimal conceptualization of either two subcomponents or a general common factor for each TPB concept within an omnibus model. Further, to test whether conceptualizations may differ by population even within the same behavioural domain, we examined these research questions with 300 undergraduates (M age = 20) and 272 cancer survivors (M age = 61) for exercise behaviour. Results identified that a general subjective norm factor was an optimal predictive conceptualization over two separate injunctive and descriptive norm components. In contrast, a specific self-efficacy component, and not controllability or a general factor of PBC, predicted intention optimally for both samples. Finally, optimal models of attitude differed between the populations, with a general factor best predicting intention for undergraduates but only affective attitude influencing intention for cancer survivors. The findings of these studies underscore the possibility for optimal tailored interventions based on population and behaviour. Finally, a discussion of the theoretical ambiguity of the PBC concept led to suggestions for future research and possible re-conceptualization.