By Matthew Warren

It’s a trick that politicians have long exploited: repeat a false statement often enough, and people will start believing that it’s true. Psychologists have named this phenomenon the “illusory truth effect”, and it seems to come from the fact that we find it easier to process information that we’ve encountered many times before. This creates a sense of fluency which we then (mis)interpret as a signal that the content is true.

Of course, you might like to believe that your particular way of thinking makes you immune to this trick. But according to a pre-print uploaded recently to PsyArXiv, you’d be wrong. In a series of experiments, Jonas De keersmaecker at Ghent University and his collaborators found that individual differences in cognition had no bearing on the strength of the illusory truth effect.

The researchers wondered whether three aspects of cognition, already known to influence how people make judgments, could determine how prone someone is to the illusory truth effect: cognitive ability or intelligence; the need for cognitive closure (i.e. the desire to avoid ambiguity); and cognitive style (whether someone thinks in a rapid and intuitive manner or takes a slower and more analytic approach). For example, someone who relies more on intuition and wants hard-and-fast answers might be more likely to use the fact that information has been repeated as a cue to its truthfulness.

Across six experiments involving between 199 and 336 participants, the team measured the illusory truth effect while also tapping into these aspects of cognition. The exact methods varied for each study, but generally participants would first read a mix of true and false trivia statements, then complete various cognitive tests and surveys, and finally they would re-read and judge as true or false the earlier trivia statements, as well as new ones interspersed among them. A seventh study was similar but involved fake and real political headlines (the participants’ final challenge in this case was to judge which were real and which were made up).

The researchers found the illusory truth effect across all seven studies: participants were more likely to rate trivia statements and headlines as true/real if they’d seen them previously. Crucially, the strength of this effect did not vary according to the participants’ cognitive ability or style, or need for closure. (A couple of studies found some small significant associations, but these disappeared when the researchers integrated all the data.)

These results suggest that we are all predisposed to believe repeated information regardless of our own particular cognitive profile. And while that might make us all susceptible to advertising and the fabrications of dishonest politicians, the researchers have a more optimistic take. “These novel findings are in line with the assertion that processing fluency is not a judgmental bias and flaw in the individual, but rather a cue to truth that is universal and epistemologically justified in most contexts”, they write. In other words, it’s not that there’s a foolish subgroup of people who are more vulnerable to the “illusory truth” effect, but rather it’s an advantageous and universal bias that’s arisen because most of the time fluency actually is a reliable signal of truth. For example, a statement that is often repeated may tend to be endorsed by more people, which could be a useful cue to its truth.

But that doesn’t mean that there aren’t individual differences relating to the illusory truth effect waiting to be discovered, the team adds. Individuals with Alzheimer’s disease, for example, seem to show a less strong effect, suggesting that certain fundamental aspects of memory and cognition may be required to support the effect. With a greater willingness to publish null results like this one – and not just leave them in the file drawer – researchers should be able to build up a much more complete picture of the illusory truth effect and other cognitive biases.

—Investigating the robustness of the illusory truth effect across individual differences in cognitive ability, need for cognitive closure, and cognitive style [This study is a preprint meaning that it has not yet been subject to peer review and the final published version may differ from the version on which this report was based]

Matthew Warren (@MattbWarren) is Staff Writer at BPS Research Digest