The common factor of conspiracy mentality: a bifactor analysis of four short-form measures of general conspiracy thinking.

Introduction: Conspiracy theory researchers increasingly rely on general measures of "conspiracy mentality" to explain belief in specific conspiracy theories. A growing consensus holds that the leading short-form scales are effectively interchangeable. This recommendation assumes that the scales measure the same underlying trait. We conduct a three-step test to determine whether four widely used measures are interchangeable in reflecting a single common conspiracy-mentality factor. Methods: Using a nationally representative U.S. sample, we conducted a bifactor analysis partitioning the common and unique variance of the American Conspiracy Theories Scale (ACT), the Conspiracy Mentality Questionnaire (CMQ), the General Measure of Conspiracism (GMC), and the Generic Conspiracist Beliefs Scale (GCB-5). We compared the general and scale-specific factors on reliability and on structural, convergent, discriminant, and criterion validity, including prediction of belief in individual conspiracy theories. Results: A single common factor accounted for most of the variance across all four short-form scales and evidenced comparable performance in terms of internal consistency, test-retest reliability, and in structural, convergent, discriminant, and criterion validity. The GCB-5 retained meaningful but apparently additional paranormal content even after controlling for the common factor of conspiracy mentality. The GMC's unique residual correlated with partisan-coded beliefs about vaccines, SARS (COVID-19), climate change, and the Obama "birther" conspiracy. The GMC also tended to produce less extreme values than the other three scales at both ends of the trait, diverging from the common factor by over a standard deviation. Discussion: The four short-form generic scales are not strictly interchangeable. While these scales share a common conspiracy mentality factor, two of them carry measurable, orthogonal content that inflates their associations with beliefs in specific conspiracy theories: paranormal content in the GCB-5, and partisan content in the GMC. The GMC also maps observed scores onto the common factor differently from the other three scales, particularly at the extremes of the trait. When measuring conspiracy thinking, researchers should treat the choice of scale as a deliberate decision and should not assume estimates obtained with different scales are equivalent. Further, we recommend that researchers consider controlling for paranormal thinking or partisanship when using the GCB-5 or the GMC, respectively.