Adaptation of utility functions to reward distribution in rhesus monkeys

This study investigated how the experience of different reward distributions would shape the utility functions that can be inferred from economic choice. Despite the generally accepted notion that utility functions are not insensitive to external references, the exact way in which such changes take place remains largely unknown. Here we benefitted from the capacity to engage in thorough and prolonged empirical tests of economic choice by one of our evolutionary cousins, the rhesus macaque. We analyzed data from thousands of binary choices and found that the animals' preferences changed depending on the statistics of rewards experienced in the past (up to weeks) and that these changes could reflect monkeys' adapting their expectations of reward. The utility functions we elicited from their choices stretched and shifted over several months of sequential changes in the mean and range of rewards that the macaques experienced. However, this adaptation was usually incomplete, suggesting that - even after months - past experiences held weight when monkeys' assigned value to future rewards. Rather than having stable and fixed preferences assumed by normative economic models, our results demonstrate that rhesus macaques flexibly shape their preferences around the past and present statistics of their environment. That is, rather than relying on a singular reference-point, reference-dependent preferences are likely to capture a monkey's range of expectations.