Previous research suggests that unfounded beliefs (UB)--such as conspiracist beliefs and beliefs in the supernatural--stem from similar cognitive and motivational mechanisms. More specifically, it has been demonstrated that cognitive ability is negatively associated with UB but only among individuals who value epistemic rationality. The present study goes beyond previous correlational studies by examining whether the negative association between cognitive ability and UB can be strengthened through a subtle rationality prime. In a large scale online experiment (N = 762 French teachers), we demonstrate that priming rationality (vs. control) does enhance the negative relationship between cognitive ability and adherence to supernatural beliefs, as well as conspiracy mentality (d = 0.2). This effect was not obtained for illusory pattern perception. This study's usefulness as a "proof of concept" for future interventions aimed at reducing UB prevalence among the general public is discussed.