The concept of justice and fairness might be hardwired as part of our social behaviour. Capuchin monkeys are very aware of fair and equal rewards, and more importantly, when underpaid they blame the payer, not the over-rewarded colleague. They can go on strike as a result. Well, withdraw their labour at least. They don’t ballot and stand by a burning barrel.
I’m very interested to see what happens when other animals are tested, and it would also be interesting to see how human children respond.