Long-tailed macaques: an unfairness model for humans

ABSTRACT

The current study was designed to predict why human primates often behave unfairly (equity aversion) by not exhibiting equity preference (the ability to equally distribute outcomes 1:1 among participants). Parallel to humans, besides inequity aversion, lab monkeys such as kin of long-tailed macaques (Macaca fascicularis) also demonstrate equity aversion depending on their preference for the outcome (food) type. During the pre-experiment phase, a food-preference test was conducted to determine the most preferred income per individual monkey. Red grapes were the most preferred outcome (100%) when compared to vanilla wafers (0%). The first set of experiments used a 1:1 ratio (equity condition) of grape distribution among six kin-pairs of female long-tailed macaques, and we compared their aversion (Av) versus acceptance (Ac). In the second experiment, we assessed the response to the 0:2 and 1:3 ratio distribution of grapes (inequity condition). A total of 60 trials were conducted for each condition with N = 6 pairs. Our results show aversion to the inequity conditions (1:3 ratios) in long-tailed macaques was not significantly different from aversion to the equity conditions (1:1 ratios). We suggest that the aversion observed in this species was associated with the degree of preference for the outcome (food type) offered rather than the distribution ratio. The subjective preferences for outcome types could bring this species into irrationality; they failed to share foods with an equal ratio of 1:1.

KEYWORDS:

• Equity aversion
• ratio
• food type preference
• long-tailed macaques

Acknowledgments

We are grateful to the Primate Research Center Institutional Grant for Post Graduate Researchers of IPB University-Indonesia and the Institutional Animal Care and Use Committee of IPB University (ACUC) with No. IPB PRC-18-B006. Thanks to Permanawati, Wahyu Putriyani, Ika Rina, Adinda Darayani Azhar as our vets and animal takers. Thanks to Vincentius Arca Testamenti for being the second observer. Thanks to Daryanto, Hendra, Udin, and Janurman as our cage technicians. Thanks to ACUC members: Fitriya N Dewi, Irma H Suparto, Maryati, and Hendra Adijuwana (Otje). Special thanks to Maginnis Gwendalyn who introduces us to the journal and Valerie Chalcraft for Chi-square opinions. Special thanks to Linda Chikita as the illustrator. Special thanks to Dwi Anto Setyawan for the video camera. Special thanks to my love Elena Della (Amy) for her support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Data availability statement

The authors confirm that the data supporting the findings of this study were available within the article and its supplementary material.

Highlights

• The idea of fairness based on inequity aversion was unreliable. Inequity aversion did not mean accepting equity. Monkeys could be averse to both inequity and equity conditions at the same time.

• Both inequity and equity aversion could happen due to monkeys did not care about the number of preferred food outcomes.

• Aversion here was defined by all negative responses including avoidance (not taking the food, throwing the tray) and stealing the partner’s food.

• We conclude that a sense of fairness was a minority in primates, and we found that these monkeys’ behavior was comparable with human behavior. Long-tailed macaques were good models of unfairness for humans.

Additional information

Funding

This work was supported by the Primate Research Center Institutional Grant for Post Graduate Researchers of IPB University-Indonesia.