Does perceptual disfluency affect the illusion of causality?

When a subjective experience of difficulty is associated with a mental task, people tend to engage in systematic and deliberative reasoning, which can reduce the usage of intuitive and effortless thinking that gives rise to cognitive biases. One such bias is the illusion of causality, where people perceive a causal link between two unrelated events. In 2019, Díaz-Lago and Matute found that a superficial perceptual feature of the task could modulate the magnitude of the illusion (i.e., a hard-to-read font led to a decrease in the magnitude of the illusion). The present study explored the generalisability of the idea that perceptual disfluency can lead to a decrease in the magnitude of the illusion. In the first experiment, we tested whether a physical-perceptual manipulation of the stimuli, specifically the contrast between the written text and the background, could modulate the illusion in a contingency learning task. The results of the online experiment (N = 200) showed no effect of contrast on the magnitude of the illusion, despite our manipulation having successfully induced task fluency or disfluency. Building upon this null result, our second experiment (N = 100) focused on manipulating the font type, in an attempt to replicate the results obtained by Díaz-Lago and Matute. In contrast to their findings, we found no discernible effect of font type on the magnitude of the illusion, even though this manipulation also effectively induced variations in task fluency or disfluency. These results underscore the notion that not all categories of (dis)fluency in cognitive processing wield a modulatory influence on cognitive biases, and they call for a re-evaluation and a more precise delineation of the (dis)fluency construct.