The desirability bias in predictions under aleatory and epistemic uncertainty.

The desirability bias (or wishful thinking effect) refers to when a person's desire regarding an event's occurrence has an unwarranted, optimistic influence on expectations about that event. Past experimental tests of this effect have been dominated by paradigms in which uncertainty about the target event is purely stochastic-i.e., involving only aleatory uncertainty. In six studies, we detected desirability biases using two new paradigms in which people made predictions about events for which their uncertainty was both aleatory and epistemic. We tested and meta-analyzed the impact of two potential moderators: the strength of evidence and the level of stochasticity. In support of the first moderator hypothesis, desirability biases were larger when people were making predictions about events for which the evidence for the possible outcomes was of similar strength (vs. not of similar strength). Regarding the second moderator hypothesis, the overall results did not support the notion that the desirability bias would be larger when the target event was higher vs. lower in stochasticity, although there was some significant evidence for moderation in one of the two paradigms. The findings broaden the generalizability of the desirability bias in predictions, yet they also reveal boundaries to an account of how stochasticity might provide affordances for optimistically biased predictions.