Assessing conspiracist ideation among Democrats and Republicans: Testing the measurement invariance of four short-form measures of conspiracist belief

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 (Democrats = 351; 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 and CMQ, 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. The ACTS only achieved the fourth-highest level (i.e., configural invariance). These results suggest that researchers who are interested in comparing conspiracist ideation between Democrats and Republicans may be best served by using the GMC.