Comparison of independence axiom violations across primates: humans and monkeys.

The Independence Axiom (IA) of Expected Utility Theory (EUT) is crucial for defining the value representation of risky choice options. With the IA, EUT presents the first of many crucial tests for utility maximization and thus serves as a benchmark for consistent choice under risk. However, the IA is frequently violated, most likely due to subjective weighting of reward probabilities. IA violations have been reported separately in humans and monkeys, but differences in payoff structure compromised comparisons. To overcome these limitations, we used matching payoff structures for the two primate species. Choices in both humans and monkeys violated the IA in a broad range of reward probabilities. In some human participants, the IA violations correlated with those of the monkeys, which indicates a common underlying decision mechanism. In other cases, the IA violations differed between the two species and reflected differences in utility functions: utility was mostly concave in humans but convex in monkeys. These inter-species comparisons provide a basis for future behavioral and neuronal studies addressing the origin of similarities and differences in primates' decisions under risk.