This is an old video but still funny and true as I bumped into it today. People often complain of unequal treatment in the workplace as a n example where people doing the same work should get similar rewards. However, some people tend to underestimate how disruptive it can be when people find out another person is getting more as an example.
In this funny experiment a pair of monkeys were used where if they gave the tester a rock, in return they would get a piece of food. They were using cucumbers and grapes where clearly the monkeys preferred grapes. So when one monkey saw that he was getting cucumbers and the other was given grapes you can see the outrage.
If an animal is smart enough to recognize inequality imagine how people react and the importance of trying to keep things fair to maintain a peaceful and productive environment.