Research on political psychology has benefitted from using individual-difference measures to predict political attitudes and behavior. And, previous research has further identified a number of specific variables that enhance the predictive utility of individual-difference scales. However, a potentially important factor that has been overlooked is the certainty with which people make their responses. The present research establishes that the certainty with which people respond to scales is associated with stability of the scale responses over time and correspondence between the scale and related outcomes. In addition, the effects of certainty in political ideological identification in predicting politically relevant outcomes hold when controlling for a number of previously established moderators. This research suggests that measuring certainty in answers to individual-difference scales can be a useful and efficient way to increase the predictive utility of those scales within the political domain and beyond. This benefit is demonstrated with need for cognition (Study 1), need to evaluate (Study 2), and ideological identification (Studies 2 and 3).