Repetition increases belief in implausible statements more than for plausible statements

The finding that repetition causes increased belief - termed the illusory truth effect - has raised important concerns about the possibility of repetition leading to widespread belief in falsehoods. This concern implies that the illusory truth effect meaningfully impacts statements that are initially thought to be implausible, as is commonly the case for falsehoods. Using a large (N = 933) online experiment, we test the relationship between the illusory truth effect and prior belief in trivia statements. In contrast to previous work, we measure the prior belief that an individual assigns to a specific statement to allow for a precise estimation of this relationship. Concerningly, we find that not only does the illusory truth effect appear meaningful for statements where participants have low prior belief, but the effect is actually stronger for these initially implausible statements. Our findings suggest that repetition can be particularly harmful when statements are initially seen as implausible.