The evolution of cooperation is one of the most important transitions in evolutionary history. The factors favoring cooperation over selfish interests have kept biologist busy over the last century. This “old” paper examined “inequality aversion” or the “sense of fairness” in brown capuchin monkey as a potential factor promoting group cohesion and cooperation.
The experiment was rather simple. The researchers challenged two monkeys (who could observe each other) with a simple task, but rewarded them unequal. One monkey consistently got a low quality reward, while the other one always got a high quality reward for exactly the same amount of work. Watch what happens!
the paper can be found here
the video can be found here