Objectives: Previous research documented that the construct of intentions related to physical activity participation by a linear function. As a consequence, researchers using linear analysis tacitly conclude that effects of unfavourable and favourable intentions on physical activity participation are exactly the same. In this study, we examined whether favourable and unfavourable intentions exerted differential effects on participation in physical activities across 13 published or unpublished studies. Method: Data consisted of 13 samples sourced from published and unpublished studies. Results: In partial support of our hypothesis, non-linear analysis revealed that in 7 out of 13 studies intentions predicted physical activity participation when intentions were favourable, but when intentions were unfavourable effects of intentions on physical activity participation were smaller. Conclusions: The theoretical significance of the present study is that it identifies a new boundary condition for the construct of intentions that delineates the more specific conditions under which intentions are more likely to predict participation in physical activities.