The unlikelihood effect: When knowing more creates the perception of less.

People face increasingly detailed information related to a range of risky decisions. To aid individuals in thinking through such risks, various forms of policy and health messaging often enumerate their causes. Whereas some prior literature suggests that adding information about causes of an outcome increases its perceived likelihood, we identify a novel mechanism through which the opposite regularly occurs.

Across seven primary and six supplementary experiments, we find that the estimated likelihood of an outcome decreases when people learn about the (by- definition lower) probabilities of the pathways that lead to that outcome. This “unlikelihood” bias exists despite explicit communication of the outcome’s total objective probability and occurs for both positive and negative outcomes. Indeed, awareness of a low-probability pathway decreases subjective perceptions of the outcome’s likelihood even when its addition objectively increases the outcome’s actual probability. These findings advance the current understanding of how people integrate information under uncertainty and derive subjective perceptions of risk.