Bonobos refuse to participate when faced with unequal rewards, reinforcing the highly contentious debate about inequity aversion in animal
To the point
- Bonobos show signs of inequity aversion: In two experiments, bonobos were less willing to participate when they received a worse reward than a partner, suggesting they dislike unequal treatment.
- Their response cannot be explained by disappointment in the experimenter: Unlike chimpanzees, bonobos still reacted negatively to receiving less than a partner when a machine - not a human - gave out the rewards, ruling out the idea that they were just disappointed with the experimenter.
- Close relationships reduce negative responses to unequal treatment: Bonobos were more tolerant of inequality when the partner receiving the better reward was someone they groomed more with, showing social bonds can influence fairness reactions.
The question of whether non-human animals have a sense of fairness has been widely debated. Some studies suggest that primates and other cooperative species show an aversion to inequity, while others argue that responses to unequal rewards can be explained by frustration, social disappointment, or food expectations. To better understand the evolutionary origins of fairness, researchers from the Max Planck Institute for Evolutionary Anthropology and Utrecht University therefore investigated inequity aversion in bonobos, one of humans' closest relatives.
The study was conducted at the Wolfgang Köhler Primate Research Centre at the Leipzig Zoo with six bonobos. The researchers conducted two rounds of experiments using established inequity paradigms: one in which the bonobos exchanged tokens for rewards, and another in which they tested whether their reactions might have been caused by disappointment in the experimenter.
Bonobos refused to cooperate if treated unfairly
The results showed that bonobos refused to participate more often when they received a lesser reward than their partner. Unlike chimpanzees, their responses could not be explained by disappointment alone, supporting the idea that bonobos genuinely recognise and respond to unfair treatment. "We found that bonobos more often refused to participate when faced with inequitable rewards," explains first author Kia Radovanović from Utrecht University. "Unlike chimpanzees, their reactions were not simply due to disappointment with the experimenter, but reflected a genuine aversion to unequal treatment."
Interestingly, bonobos were more tolerant of inequity when interacting with closely affiliated partners, similar to patterns seen in humans. This suggests that social bonds may influence reactions to fairness, reinforcing the idea that inequity aversion has evolved as a stabilizing force for cooperation. "This study provides new insights into how fairness may have evolved, but further research is needed," says Daniel Haun, Director at the Max Planck Institute for Evolutionary Anthropology. "Testing larger groups of bonobos and examining inequity aversion in naturalistic cooperation settings will be key to refining our understanding."
The findings contribute to the ongoing debate on inequity aversion and highlight bonobos as a unique model for studying the evolution of fairness.
