There are few markers that make humans unique and separate from other animals. While things like the ability to use tools, hunt, and even form culture have all been found in other creatures, our social structure and behavior are still seemingly a one-off. The darker side of our societies, such as the ability to be spiteful, were also thought to be unique, until now. It seems that capuchin monkeys can be spiteful to each other, even it means that both parties end up with nothing. 

“One hallmark of the human species is the fact that we’re willing to make a special effort to punish those who violate social norms,” explains Laurie Santos, senior author of the study published in Evolution and Human Behavior, in a statement. “We punish those who take resources unfairly and those who intend to do mean things to others. Many researchers have wondered whether this motivation is unique to our species.”

And it turns out that it’s not. In an experiment where a capuchin monkey could decide whether or not another individual gets food by pulling a rope that makes the table on which the food sits collapse, they found that the monkeys seemingly pull the cord out of spite. When conducted on chimpanzees, they found that the apes would only pull the cord out of a sense of revenge – for example, if the other ape stole some of their food. But not so for the capuchins. They pulled the rope, depriving the other monkey of food even though it meant that they both went without.  

^{Chimps live in highly complex social groups, but whether or not they show true altruism is still debated. scandium/Shutterstock}

“Our study provides the first evidence of a non-human primate choosing to punish others simply because they have more,” said Kristin Leimgruber, lead author of the paper. “This sort of ‘if I can't have it, no one can’ response is consistent with psychological spite, a behavior previously believed unique to humans.”

Researchers have long puzzled over how altruistic behavior and empathy have evolved in social species and why it is so developed in humans. Experiments involving chimpanzees, our closest living relatives, have shown that they probably do show empathy between each other, as documented when chimps will groom individuals who were the “victim” of fights. Not only do they show it within their species though, they potentially show it between species too. Using yawning as a measure of empathy, researchers have shown how chimps seem to show the behavior towards humans, even ones unfamiliar to the apes, but not towards baboons.

Altruism is a difficult behavior to explain in the natural world, as a truly selfless act is often costly to the individual performing it. If there is any gain to the original animal, even if it is delayed down the road, then it’s not truly altruistic. But the experiment involving the capuchins showed the flipside of this, and instead demonstrated the monkeys performing an entirely spiteful act, one in which neither party gained, something previously thought to be limited to mankind.