Re-Evaluating the Role of Simplicity in Explanation Evaluations

People often prefer simpler explanations, sometimes even beyond what is justified by available probability information, creating a form of simplicity bias. This work investigates the source and nature of such simplicity biases. Across three studies and four supplementary studies, the current work contrasts the dominant Bayesian account, where an explanation’s structural simplicity (number of root causes) is used as a cue to its prior probability, with an alternative compression-based account, which emphasizes the key role of cognitive simplicity (ease of processing, or processing fluency). Results consistently support the latter view. Simplicity biases were closely tied to processing ease, emerging from both spontaneous variation (Study 1) and from manipulations of explanations’ clarity and familiarity (Study 2), even when holding the number of causes constant. Simplicity biases also arose directly in posterior probability judgments, rather than in priors—consistent with a compression-based view, if the main processing difficulty arises when linking complex explanations with their effects—while priors instead showed a complexity bias, due to overestimating joint probabilities. Study 3 shed light on the functions of these simplicity biases, suggesting that ease of processing is seen both as a cue to an explanation’s probability, and as a valued feature in its own right. Together, these findings suggest that biases towards simpler explanations reflect general mechanisms related to processing ease, rather than a distinct role for structural simplicity, and highlight the importance of considering cognitive processes, not just abstract properties of explanations, in order to understand both mechanisms and biases in explanatory reasoning.