Measuring conspiracy beliefs among Democrats and Republicans: A test of the measurement invariance of four short-form conspiracist ideation scales

Are Republicans more likely to believe in conspiracy theories than Democrats? This question has received considerable attention among researchers, but answering it requires measures of conspiracist belief that function the same among Democrats and Republicans. Unfortunately, evidence of such measurement invariance is scarce. To address this limitation, the current preregistered study (n_Democrats = 351; n_Republicans = 339) tested the invariance of four short-form conspiracist ideation measures—the General Measure of Conspiracism (GMC), the Generic Conspiracist Beliefs Scale – 5 (GCB-5), the Conspiracy Mentality Questionnaire (CMQ), and the American Conspiracy Thinking Scale (ACTS)—across the two major US political parties. Given the lack of prior research on the topic, we preregistered the optimistic hypothesis that all four scales would achieve the highest level of invariance (i.e., strict factorial invariance). The GMC was the only measure to reach this level. The GCB-5, CMQ, and ACTS, by contrast, only achieved the third-highest level (i.e., metric invariance), despite the GCB-5 demonstrating the greatest overall fit of the measures tested. Researchers who are interested in comparing levels of conspiracist ideation between Democrats and Republicans may, therefore, be best served by using the GMC.