Dispelling illusions of truth: exploring the factors that lead to inflated truth judgements

Judging the truth of incoming information is one of the most challenging and important tasks that people face every day. How do people decide what is true and what is not? When constructing truth judgements, people use both declarative information and the subtler cues that accompany information processing. These subtle, non-content-based cues that make information feel truer are termed ‟truth effects”. This thesis uses trivia statements to investigate the robustness of two such non-probative truth effects driven by repetition (the illusory truth effect) and concrete language (the linguistic concreteness effect). Neither concreteness nor repetition provide substantive evidence, yet people believe repeated statements more than new ones, and concretely worded statements feel truer than their abstract counterparts. Truth effects can have direct implications in our digital world, where information may be spurious, and communicators can enlist subtle cues to persuade the addressee without detection. Throughout the thesis I apply open methods that have the potential to increase the quality, replicability, and transparency of research. In Chapter 2, I set out to replicate and extend the linguistic concreteness effect. Across two experiments I did not observe an effect larger than the smallest effect size of interest. Therefore the remainder of the thesis focuses on the illusory truth effect. Chapter 3 uses systematic mapping to synthesise and catalogue the entire illusory truth literature in terms of methods, findings, and transparency. The results reveal a lack of standardisation in the methodology employed, and of transparency in reporting. I also find that greater diversity of stimuli and participants is required for generalisability. In Chapter 4, my final study used a longitudinal design to test whether the delay between repetitions moderates illusory truth. Contrary to previous claims, I find that across four intervals (immediately, one day, one week, one month) the effect diminishes as delay increases. This thesis contributes to knowledge by providing an overview of the current state of truth effects research. It demonstrates that there is considerable cause to doubt the existence of a linguistic concreteness effect, and by implication, there is reason to be sceptical about other truth effects based on subtle manipulations. In contrast, this thesis establishes confidence that the illusory truth effect is robust but reduces with time. This finding has implications for the mechanisms thought to underlie truth effects. Overall, the results suggest that when truth effects research uses rigorous, transparent, and unbiased methods, it paints a different picture from that of the existing literature.