Life isn’t fair.

It’s a phrase so often repeated that it has become a cliché. But studies have shown that humans are hard-wired to want their fair share, as are other animals that have cooperative relationships, like monkeys, birds and wolves.

In one famous experiment, researchers trained two capuchin monkeys to hand them tokens in exchange for a cucumber snack. At first, the animals were happy with this arrangement — that is, until one of the monkeys received grapes instead, which are considered far more delicious. The other monkey, who continued to receive cucumbers, looked enraged, shook the walls of her enclosure and hurled the cucumbers out of reach. READ MORE