Repetition could increase the perceived truth of conspiracy theories.

Conspiracy theories can be encountered repeatedly, which raises the issue of the effect of their repeated exposure on beliefs. Earlier studies found that repetition increases truth judgments of factual statements, whether they are uncertain, highly implausible, or fake news, for instance. Would this "truth effect" be observed with conspiracy statements? If so, is the effect size smaller than the typical truth effect, and is it associated with individual differences such as cognitive style and conspiracy mentality? In the present preregistered study, we addressed these three issues. We asked participants to provide binary truth judgments to conspiracy and factual statements already displayed in an exposure phase (an interest judgment task) or that were new (displayed only in the truth judgment task). We measured participants' cognitive style with the three-item Cognitive Reflection Test (CRT), and conspiracy mentality with the Conspiracy Mentality Questionnaire (CMQ). Importantly, we found that repetition increased truth judgments of conspiracy theories, unmoderated by cognitive style and conspiracy mentality. Additionally, we found that the truth effect was smaller with conspiracy theories than with uncertain factual statements, and suggest explanations for this difference. The results suggest that repetition may be a simple way to increase belief in conspiracy theories. Whether repetition increases conspiracy beliefs in natural settings and how it contributes to conspiracism compared to other factors are important questions for future research.