A recent experiment challenges the belief that justice is a uniquely human trait. The German Center for Primatology reports new findings that undermine the idea of a built in sense of fairness in monkeys.
For many years, researchers assumed that only people could feel complex emotions such as injustice. Understanding this concept was thought to require a highly developed brain, the ability to compare oneself with relatives, and the skill to foresee how actions lead to rewards or punishments. In recent times, however, hints have appeared suggesting that monkeys may react to fairness in some way. A widely shared example showed a monkey rejecting a cucumber reward offered by a trainer when another animal received sweeter grapes for completing the same task.
New insights come from a study of cynomolgus monkeys, which proposes an alternative explanation for such reactions that does not rely on advanced abstract thinking. The investigation used four scenarios that shared a common structure: a monkey received a poor reward for a task that another animal received a higher quality reward. In two scenarios, a human delivered the reward, in two it came from an automatic feeder, and in two it involved a peer and a monkey as the recipient. Across all setups, the relative with the better prize always appeared in the same way, allowing four distinct experimental combinations to be tested.
The results showed that monkeys almost never reject a subpar reward when it is dispensed by an automatic feeder, but they did refuse the low reward in more than one in five trials when a person was the reward giver. This pattern aligns with a common human response to perceived unfairness when a person acts with bias or inconsistency. It also appears that monkeys can distinguish between living beings and inanimate mechanisms, as they did not react strongly to the behavior of the person delivering the reward. The presence of a relative receiving a superior reward did not significantly influence the choice to reject or accept a reward. Interestingly, strong gender differences emerged in the data: in the trials where a woman delivered the reward and a man was the recipient, rejection occurred in about 8% of cases, compared with 42% in the opposite arrangement.
From these observations, researchers conclude that what was once described as a rebellion against injustice in earlier experiments may instead reflect an individual response to perceived unfair treatment by a person. Nevertheless, the researchers caution that further work is needed to confirm these findings, given the complexity of such studies and the number of small variables that must be controlled in future trials.
These results emphasize the importance of carefully distinguishing genuine moral judgments from more basic reactions to unequal outcomes. They suggest that what looks like a quest for justice in nonhuman primates could be driven by simpler cognitive processes tied to social dynamics, reward expectations, and the mechanics of reward distribution. The ongoing exploration will continue to refine our understanding of primate behavior and the roots of fairness across species. [German Center for Primatology]