Researchers taught brown capuchin monkeys to swap tokens for cucumber.
But if one monkey got given a grape, which they prefer, for the same token, the other monkeys got really upset.
They refused to work, or didn't eat the cucumber they'd been given.
But Sarah Brosnan, who was in charge of the research, said the monkeys never took it out on one another.
"They never showed a reaction against their partner, they never blamed them," she said.
Scientists reckon this is important because it could show that our sense of justice doesn't just come from society, but is in our genes.