Rhesus monkeys lack a consistent peak-end effect

Abstract

In humans, the order of receiving sequential rewards can significantly influence the overall subjective utility of an outcome. For example, people subjectively rate receiving a large reward by itself significantly higher than receiving the same large reward followed by a smaller one (Do, Rupert, & Wolford, 2008). This result is called the peak-end effect. A comparative analysis of order effects can help determine the generality of such effects across primates, and we therefore examined the influence of reward-quality order on decision making in three rhesus macaque monkeys (Macaca mulatta). When given the choice between a high–low reward sequence and a low–high sequence, all three monkeys preferred receiving the high-value reward first. Follow-up experiments showed that for two of the three monkeys their choices depended specifically on reward-quality order and could not be accounted for by delay discounting. These results provide evidence for the influence of outcome order on decision making in rhesus monkeys. Unlike humans, who usually discount choices when a low-value reward comes last, rhesus monkeys show no such peak-end effect.

Keywords:

• Decision making
• Peak-end rule
• Order preferences
• Contrast effects
• Rhesus monkeys

Acknowledgments

Our funding was provided by the National Institute of Mental Health's Research Career Award for Transition to Independence (K22) grant (1 K22 MH071756–01), under the title “Neurophysiology of Inhibitory Control in Decision Making” with principal investigator Jerald D. Kralik. The funding source was not involved with any aspect of this study or its submission. We thank Sara Kahn, William Levine, and William Sampson for helping with data collection. We would also like to thank Steven Wise for helpful discussions and comments on an earlier version of the manuscript.