Adaptation of utility functions to reward distribution in rhesus monkeys

This study investigated the influence of experienced reward distributions on the shape of utility functions inferred from economic choice. Utility is the hypothetical variable that appears to be maximized by the choice. Despite the generally accepted notion that utility functions are not insensitive to external references, the exact occurrence of such changes remains largely unknown. Here we benefitted from the capacity to perform thorough and extensive experimental tests of one of our evolutionary closest, experimentally viable and intuitively understandable species, the rhesus macaque monkey. Data from thousands of binary choices demonstrated that the animals’ preferences changed dependent on the statistics of recently experienced rewards and adapted to future expected rewards. The elicited utility functions shifted and extended their shape with several months of changes in the mean and range of reward distributions. However, the adaptations were usually not complete, suggesting that past experiences remained present when anticipating future rewards. Through modelling, we found that reinforcement learning provided a strong basis for explaining these adaptations. Thus, rather than having stable and fixed preferences assumed by normative economic models, rhesus macaques flexibly shaped their preferences to optimize decision-making according to the statistics of the environment.