In the best-known clip from this experiment, a capuchin monkey hands a small stone to a researcher and gets a slice of cucumber in return. It eats the cucumber happily. Then it watches the monkey in the next enclosure do exactly the same thing and receive a grape. On the next round, the first monkey takes its cucumber, looks at it and throws it straight back at the researcher, then rattles the wall of its enclosure. It is funny, a little uncomfortable and surprisingly familiar to anyone who has found out that a colleague earns more for the same job.
The video comes from research by the primatologists Sarah Brosnan and Frans de Waal at Emory University in Atlanta. De Waal showed the footage in his TEDxPeachtree talk "Moral behavior in animals", and the short excerpt later spread online under titles about monkeys being "paid unequally".
What happened in the cucumber and grape experiment
Brosnan and de Waal published the original study in the journal Nature in 2003. They worked with brown capuchin monkeys that had been trained in a simple token exchange: a monkey gives a pebble back to the experimenter and receives food. Capuchins will happily eat cucumber, but they clearly prefer grapes.
Two monkeys were tested side by side so that each could see what the other did and what it received. The researchers compared four situations:
- Equal pay: both monkeys did the task and both got cucumber.
- Unequal pay: both did the task, but the partner got a grape.
- Grape for nothing: the partner received a grape without doing any work.
- Grape in view, no partner: a grape was visible, but there was no other monkey getting it.
When rewards were equal, the monkeys cooperated almost every time. When the partner got grapes for the same task, the cucumber monkey refused its food or failed to return the token far more often, and the refusals rose further when the partner got a grape for doing nothing at all. The five monkeys in the main analysis were females; in early tests the males did not show the same difference between conditions.
The famous throwing scene is in the middle of the clip, roughly from 1:33 to 2:05.
Why the study became so famous
Economists and psychologists use the term inequity aversion for the tendency to dislike outcomes that are unequal. Humans show it in many settings: in laboratory games, people often reject a small offer of free money if they feel the split is unfair, even though rejecting it leaves them with nothing.
The capuchin study suggested that a basic version of this reaction might exist in other animals, and that our sense of fairness has deep evolutionary roots rather than being purely a product of culture or law. That idea fitted de Waal's wider work. Over several decades he argued, in books such as Chimpanzee Politics, Our Inner Ape and Mama's Last Hug, that empathy, reconciliation and cooperation in primates are building blocks of human morality. The Nature paper has since been cited thousands of times.
Other animals tested since
The experiment inspired a long line of similar studies in other species, with mixed results:
- Dogs: several studies found that dogs stop giving a paw sooner when a partner is rewarded and they are not, though other studies found no clear effect.
- Chimpanzees: some experiments led by Brosnan reported reactions to unequal rewards, and one even found chimpanzees reacting when they themselves got the better deal; others, such as work by Juliane Bräuer and colleagues, found no evidence.
- Crows: a small study of six crows found sensitivity to unequal rewards, while a later crow study did not.
The pattern so far is that responses to unfair treatment appear most often in species that cooperate closely with unrelated partners, but the evidence is far from uniform.
Is it really about fairness? The main criticisms
A memorable video can make the conclusion look more certain than it is. Researchers have offered other explanations for the cucumber monkey's protest:
- Food expectation: seeing grapes nearby may simply raise a monkey's hopes, so cucumber becomes disappointing. Some teams found refusals even when a grape was visible but no partner received it.
- Frustration or contrast: the monkey may be reacting to a worse reward than the one it has just seen, rather than comparing its treatment with the other animal's. A 2020 analysis argued that capuchin rejections are better explained by this contrast effect.
- Small samples: many of these studies involve only a handful of animals, so the results can vary from group to group.
Later studies tried to control for these explanations, for example by hiding food or placing it in another cage. The debate is still open. What most researchers do agree on is that the monkeys care about what others around them receive, not only about what they get.
What the monkeys can teach us about pay and fairness
The clip is often shared as a lesson about unequal pay, and it works as an analogy as long as we remember it is one. A few practical points carry over well:
- Comparison matters as much as the reward. The cucumber was acceptable until a better reward appeared next door. People also judge their pay and conditions against what others receive.
- Visible unfairness damages cooperation. The monkeys did not just complain; they stopped taking part. Teams and workplaces can lose goodwill the same way when rewards look arbitrary.
- Transparency helps. When the reasons for different rewards are clear and fair, people are less likely to feel cheated.
The researchers behind the video
Sarah Brosnan completed her PhD at Emory in 2004 and went on to become a Distinguished University Professor of psychology at Georgia State University, where she continues to study cooperation and fairness in primates. Frans de Waal, born in the Netherlands in 1948, joined Emory in 1991, directed the Living Links Center at the Emory National Primate Research Center and retired in 2019. He died in March 2024, aged 75. Both used the cucumber and grape footage in public talks, and it remains one of the most widely recognised animal-behaviour videos online.
Sources: Wikipedia: Inequity aversion in animals (summary of Brosnan & de Waal, Nature 2003, and later studies); Emory News Center: Frans de Waal remembered; video excerpt from Frans de Waal, "Moral behavior in animals", TEDxPeachtree; originally shared via Why Don't You Try This.
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