<p>Rational Emotive Behaviour Therapy (REBT) distinguishes between ego disturbance and discomfort disturbance as key mechanisms of emotional dysfunction. The present study introduces the Stressful Belief Index (SBI), a 30-item measure evaluating Approval Demand, Demands of Others, Life Demand (discomfort disturbance), and Perfection Demand. Data were gathered from 160 clinical clients. The SBI showed a clear four-factor structure with strong internal consistency across its subscales. Convergent validity was supported through correlations with the Burns Self-Defeating Beliefs Scale. Life Demand exhibited the strongest associations with both depression and anxiety and accounted for the largest portion of variance in hierarchical regression models. When discomfort disturbance was entered before ego disturbance, ego disturbance contributed minimal additional variance; the reverse was not observed. These findings suggest that discomfort disturbance may be a more central cognitive mechanism within the REBT framework of demandingness. The results are discussed in the context of contemporary scientific models of psychological disturbance, such as Feldman Barrett’s predictive modelling approach, and overlap with the historical themes of demandingness emphasized by Albert Ellis.</p>