Humanity’s throne atop the animal kingdom is wobbling these days. Research has shown that non-human animals can make complex tools, give gifts to their friends, and even read complicated medical images. But it can’t all be good news. Some of humanity’s less desirable traits have parallels in our non-human relatives. In fact, a recent study published in Evolution and Human Behavior shows that capuchin monkeys share our capacity for acting spiteful.
We’ve all seen animals behaving in ways we could interpret as spiteful. But scientists have argued that there’s really no way to understand the motivations of, say, a dog that chews up your socks, because there are just too many variables at play.
So scientists designed an experiment that would allow them to test for spite and spite alone. “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,” senior author Laurie Santos said in a press 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.”
Capuchin monkeys (Cebus apella) are carefully attuned to one another’s behavior. Previous experiments with C. apella have shown that capuchins pay attention to how fairly or evenly resources are distributed—that is, if another monkey got more of something. When they have to complete a group task, capuchins tend to avoid teaming up with monkeys who have a track record of being uncooperative or selfish.
To complete the research, the researchers first set up collapsible tables attached to long ropes. Then, they gave the monkeys some time to learn how the rope-table setup worked. When the scientists were sure the monkeys understood the consequences of pulling the rope, the capuchins were divided into two groups: subjects and stooges.
During each test, the researchers sat one stooge monkey at the table and gave control of the rope to a subject monkey. Then the food came out. It was good food, too—Fruity Pebbles cereal (which the researchers describe as “a highly valued food reward”) and marshmallow fluff. The stakes were high.
There was no mistaking the intentions of these monkeys. Time and time again, subject monkeys would yank the rope to collapse the stooges’ table and make the food disappear. This was true even when the stooges had done nothing to get the food; in other words, the subjects punished them for getting food even when they were innocent.
“Our study provides the first evidence of a non-human primate choosing to punish others simply because they have more,” first author Kristin Leimgruber, said in the press release. “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.”