Experiments that study the behavior of social animals give us valuable insight into the innate tendencies embedded in them—including in humans—that make life in a society possible at all. One such experiment is the well-known study with monkeys receiving unequal “pay” for the same task. In short, it works like this:

Two capuchin monkeys are placed in adjacent cages with transparent walls, so they can see each other. Each monkey is asked to hand a small stone to the experimenter, taken from inside its cage, in exchange for food (initially a slice of cucumber). Both monkeys perform the task eagerly, because they like cucumber.

At a certain point, however, one of the monkeys receives not cucumber for the same effort, but grapes—which capuchins clearly prefer. The other monkey continues to receive cucumber. Upon seeing that its neighbor has been rewarded with grapes, the “underpaid” monkey angrily throws the cucumber out of the cage, refuses to continue the task, and begins to protest and rage.

You can observe this behavior in the following video:

This experiment has been repeated under different conditions and with different animals, but the results are consistently similar, varying only within certain limits. We are therefore left with two options: either social animals possess some innate sense of fairness, or the experimental subjects were somehow previously indoctrinated with communism and Marxism.

Jokes aside, while the experiment—and the video in particular—can be amusing, it very clearly illustrates the deep-seated tendencies present in social animals, tendencies upon which human morality later develops. This line of research is often associated with the work of primatologists such as Frans de Waal, who has repeatedly argued that fairness, reciprocity, and a sense of justice are not cultural inventions layered on top of human nature, but evolutionary traits with deep biological roots.

In that sense, justice may look like “monkey business”—but it is precisely this ancient, pre-rational sensitivity to fairness that makes complex social life possible at all.