Rhesus macaque choices reveal that ambiguity aversion is driven by pessimistic probability estimates.

Economic uncertainty has multiple forms, including risk and ambiguity. Risk describes lotteries with well defined probabilities, whereas ambiguity describes cases where probabilities are not known. Given choices between risky and ambiguous alternatives, most decision makers prefer the known uncertainty of risk. This relationship, known as ambiguity aversion, holds even when both options have the same quantitative levels of uncertainty. Despite the prevalence of ambiguity in the natural world, the psychological and neural mechanisms that produce ambiguity aversion are not well understood. Here, we developed a nonhuman primate task to study ambiguity attitudes in the lab. We showed that the animals are strongly ambiguity averse, even when they could learn the true values of the ambiguous options through repeated trials. Then, to understand why they were ambiguity averse, we examined points of subjective equivalence between risky gambles and ambiguous alternatives. These experiments demonstrated that when the potential outcomes were large, the animals judged the probability of getting the better outcome far below the true probability of 0.5. Interestingly, we observed the opposite in small value gambles: the animals were overly optimistic about their chances of getting the better reward, even though the true probability was still 0.5. These inconsistent judgments about probabilities suggest that the animals do not hold consistent beliefs, and that the lack of consistent beliefs leads to suboptimal economic outcomes. These results provide a platform to study the neurophysiological bases of real-world decision making.