Capuchin monkey reacts to an unfair situation
The famous cucumber-grape study on fairness, done by Brosnan and de Waal.
Two capuchins were situated in enclosures next to one another. A researcher would ask them to do a task and if they succeeded give them a treat. The catch was one monkey was always rewarded with a piece of cucumber while the other monkey sometimes got a piece of cucumber and sometimes got a grape — a preferred treat among capuchin monkeys.
A monkey that received only cucumber appears perfectly happy until she sees her companion receive a grape. Then her behavior changes. She accepts the next piece of cucumber only to throw it back at the researcher, pounding the surface in front of the enclosure and shaking its Plexiglas walls.
“That video struck home with a lot of people,” Brosnan says. “Who hasn’t felt like that monkey that’s only getting cucumbers? Our research showed something about the evolution of the sense of human fairness.”
For those wanting to dive more into the science, you can do a search in Google Scholar for "inequity aversion"