<p>Human reasoning involves a balance between fast, intuitive responses and slower, deliberative thinking. While deliberation is effortful and often unpleasant, its motivational triggers remain unclear. Drawing on appraisal theory, we propose that the emotion of doubt—characterized by uncertainty and cognitive blockage—acts as a trigger for deliberation. In this pre-registered study (<i>N</i> = 414), participants solved deductive reasoning problems varying in cognitive conflict (conflict vs. non-conflict) while being instructed to frown or not, based on the facial feedback hypothesis. Doubt and several indices of deliberation were measured after each trial. Results showed that conflict problems elicited greater doubt and arousal, and that doubt was positively associated with self-reported cognitive effort. Frowning amplified these emotional effects, increasing doubt and arousal, though it had limited impact on behavioral measures of deliberation (e.g., response times, answer changes). These findings suggest that doubt, as a negative emotional response to conflict, plays a motivational role in engaging deliberation. Frowning may modulate this process by enhancing the emotional salience of conflict. The results support an integrated model of reasoning in which emotion,particularly doubt, helps regulate the shift from intuitive to deliberative thought.</p>