Repetition increases belief in climate-skeptical claims, even for climate science endorsers

matched counterbalances containing target climate-related claims, and weather claims from Experiment 1. Inspecting either counterbalance separately did not affect the main findings. Experiment 2 followed the same procedure as Experiment 1 with two additions. After giv- ing truth ratings of all claims in the truth testing phase (4 scientist, 4 skeptic, 16 weather), par- ticipants completed SASSY [44], a well-validated measure of climate change attitudes allowing categorical analysis of climate change position/beliefs, as an alternative measure of the between-subjects covariate of Belief. Then, participants self-classified each climate-related claim as aligning with views of a climate scientist, views of a climate skeptic, or if they were unsure. Results Sample characteristics Most participants are climate science endorsers. Our key research question was whether a single repetition increases the perceived truth of a claim even when the claim runs counter to the recipient’s own climate change beliefs. Hence, we first examined the beliefs and attitudes of our participants across both Experiments 1 and 2. We calculated mean scores of the Belief in (climate change) Science (Cronbach’s alpha = .91 for Experiments 1 and 2) and Attitudes Towards (climate change) Science items (Cronbach’s alpha = .83 for Experiment 1, .81 for Experiment 2). We used participants’ mean score across both scales to represent the belief-in- climate-science variable. We found that 90% of participants in Experiment 1 and 92% in

Experiment 2 endorsed anthropogenic climate change (having mean scale scores of above 4— the middle of the 7-point scale). This characterization of our samples was supported by using SASSY in Experiment 2: of our 120 participants (including 5 uncategorized), 36 participants were Alarmed (31%), 35 Concerned (29.2%), 27 Cautious (22.5%), 0 Disengaged (0%), 8 Dis- missive (7%), and 14 Doubtful (11.7%). These samples allow us to examine how the repetition of climate skeptic-aligned claims affects the beliefs of climate science endorsers, which is cen- tral to the discussion of false-balance reporting [3–5]. However, our samples do not allow us to examine the response of climate skeptics to repetitions of climate science-aligned claims. In our main analyses, we include only people who are categorized as climate science endorsers by the belief variable (Experiment 1; N = 47) or by both the belief variable and the SASSY classification (Experiment 2; N = 110). Accordingly, skeptic claims are always counter- attitudinal claims that contradict the climate change beliefs and attitudes of our samples. Including participants who do not endorse climate science in the linear mixed models does not change our main conclusions. Illusory truth effect for counter-attitudinal claims Repetition increased truth ratings for both pro- and counter-attitudinal claims. Our main research question was whether climate science endorsers would perceive climate-skeptic claims as more likely to be true when they have seen them before. That is, can a single repeti-

tion of a counter-attitudinal claim increase its perceived truth? To answer this question, we tested whether claim type (climate scientist-aligned vs. climate skeptic-aligned) moderated the influence of repetition on judged truth by conducting a linear mixed effects model with the fol- lowing factors: participant (random), claim type (fixed; scientist-aligned vs. skeptic-aligned), repetition (fixed; repeated vs. non-repeated), and repetition x claim type interaction (fixed). We report results for each fixed effect in order below, and also present estimates of fixed effects for Experiment 1 (Table 1) and Experiment 2 (Table 2). Given that our samples were climate science endorsing, we would expect people to rate cli- mate scientist-aligned claims as truer than skeptic-aligned claims. This is what we found. PLOS ONE Illusory truth effect for climate-skeptical claims PLOS ONE | https://doi.org/10.1371/journal.pone.0307294 August 7, 2024 6 / 19