Measuring Belief in Conspiracy Theories Across Borders: A Cross- National Psychometric Approach

Despite growing interest in conspiracy theories, cross-national measurement of specific conspiracy beliefs (CB) remains underdeveloped. Existing CB scales suffer from content contamination, contextual specificity, and weak validation, especially in comparative settings, prompting most research to rely on conspiracy mentality (CM) measures. This paper introduces the Comparative Conspiracy Beliefs Scale (COMP-CB), a new instrument designed to assess non-political CB with cross-national comparability and strong psychometric properties. In Study 1, we used an inductive approach to select eight high-performing, thematically diverse items from a pool of 60 conspiracy theories, tested across eight Western countries (N ≈ 1,600). Structural Equation Modeling confirmed the scale unidimensionality, internal consistency, and metric invariance. In Study 2, we validated the COMP-CB on nationally representative samples (N ≈ 20,000), examining its associations with a broad range of political, cognitive, and psychological variables. We also compared its correlational profile to a validated scale of conspiracy mentality, the GCB-5. While both scales share common correlates, the COMP-CB is more strongly associated with belief in paranormal phenomena, support for legitimate radical political action, and favorable attitudes toward extremist political actors. In contrast, the GCB-5 is more closely linked to populist and anti-elitist orientations. These findings underscore the value of belief-based measures in capturing politically consequential dimensions of conspiracism.