[W]e did a study in which capuchin monkeys received either a grape or a piece of cucumber for a simple task.
If both monkeys got the same reward, there never was a problem. Grapes are by far preferred (as real primates, like us, they go for sugar content), but even if both received cucumber, they’d perform the task many times in a row.
However, if they received different rewards, the one who got the short end of the stick would begin to waver in its responses, and very soon start a rebellion by either refusing to perform the task or refusing to eat the cucumber.
This is an “irrational” response in the sense that if profit-maximizing is what life (and economics) is about, one should always take what one can get. Monkeys will always accept and eat a piece of cucumber whenever we give it to them, but apparently not when their partner is getting a better deal. In humans, this reaction is known as “inequity aversion.”