Human Spite Started in Monkey-Ape Ancestor
Spiteful behavior is seen to go hand in hand with cooperation among monkeys.
The behavior appears to go hand in hand with cooperation, helping to explain why the most cooperative primate species can also include those who are malicious.
"Cooperation is central to human societies, and the punishment of non-cooperators is thought to play a key role in both the emergence and maintenance of cooperation within social communities," wrote Kristin Leimgruber and her team in a study published in the journal Evolution and Human Behavior.
Leimgruber, a researcher at Yale University, and her colleagues Alexandra Rosati and Laurie Santos studied brown capuchin monkeys, which are known for their intelligence, tool use, self-awareness and cooperation.
The scientists created an experiment where the monkeys could yank on a rope to collapse a table holding a partner monkey's food. Prior research found that chimpanzees would only collapse a partner's table if they experienced a direct personal affront, like if the other chimp stole their food right in front of them. But the monkeys would punish their partner even if the other just had more to eat.
One possible psychological motivation is spite.
"While biologists typically define spite as taking an action at cost to oneself to impose a cost on another, spite at the psychological level involves a tendency to inflict suffering upon a target as a means to an end," the researchers wrote. "Although spite is typically considered unique to human cooperation, the current results are consistent with the possibility that capuchins may experience spite at the psychological level as well."
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Brown capuchin monkeys therefore seem to be capable of spiteful behavior and are sensitive to inequity, traits that together were formerly only associated with humans.
"As a result, it appears as though the evolutionary roots of some human-like punitive tendencies may extend even deeper than previously thought," the researchers believe.
If both monkeys and humans have inherited these tendencies, then they could have been present in the last common ancestor of both of these primates. While the precise identity of that ancestor remains unknown, scientists generally agree that the species lived around 35 million years ago, was small-brained and lemur-like.
It is unclear why some primate species seem to be more cooperative (and, on the flip side, more spiteful) than others, but another new study sheds light on how such behavior can emerge.
Joshua Plotkin, a professor at the University of Pennsylvania's Department of Biology, and colleague Alexander Stewart determined that cooperation emerges when groups are small and memories are long. The findings were published a few weeks ago in the journal Scientific Reports.
"I think a fascinating takeaway from our study," Stewart said, "is that you can get a set of circumstances where there is a kind of runaway feedback loop. Longer memories promote more cooperation and more cooperation promotes longer memories. That kind of situation, where you go from a simpler system to one that is more complex, is a great example of what evolution does, it leads to more and more complexity."
Can cooperation and better long-term memory continue to evolve, however, without the emergence of spiteful behaviors? So long as inequality exists, that appears doubtful.