Of tinfoil hats and thinking caps: Reasoning is more strongly related to implausible than plausible conspiracy beliefs

People who strongly endorse conspiracy theories typically exhibit biases in domain-general reasoning. We describe an overfitting hypothesis, according to which (a) such theories overfit conspiracy-related data at the expense of wider generalisability, and (b) reasoning biases reflect, at least in part, the need to reduce the resulting dissonance between the conspiracy theory and wider data. This hypothesis implies that reasoning biases should be more closely associated with belief in implausible conspiracy theories (e.g., the moon landing was faked) than with more plausible ones (e.g., the Russian Federation orchestrated the attack on Sergei Skripal). In two pre-registered studies, we found that endorsement of implausible conspiracy theories, but not plausible ones, was associated with reduced information sampling in an information-foraging task and with less reflective reasoning. Thus, the relationship between belief in conspiracy theories and reasoning is not homogeneous, and reasoning is not linked specifically to the “conspiracy” aspect of conspiracy theories. Instead, it may reflect an adaptive response to the tension between implausible theories and other beliefs and data.