We may have finally found a behavior that sets humans apart from chimpanzees. We all display empathy and use tools, but according to Katrin Riedl and her colleagues from the Max Planck Institute for Evolutionary Anthropology, only humans will punish transgressors who don’t personally harm them. This is known as ‘third-party punishment’ and is the hallmark of the human justice system.
When one chimp steals food from another, the victim will usually attempt to retaliate, unless ranking prevents this. But what happens if a chimp witnesses one chimp (thief) stealing from a third chimp (victim)? The observer has no personal stake in the outcome. Will he intervene or punish the transgressor? Apparently not.
The researchers gave thirteen chimpanzees (referred to as the ‘actors’) the chance to watch their fellows stealing and being stolen from. Of course, the animals in the study were not allowed to snatch food and wantonly attack each other. They were placed in cages where they could see each other and given the opportunity (in some cases) to drag food towards themselves using ropes. Actors could cause the food platform to collapse, depriving the thieves (or the victims if they so chose) of the goods.
While the actors readily retaliated when their own food was purloined, they could not be bothered to step in when the crime was committed against another individual. This was true even when the actor was dominant and thus a ‘community leader’, or when the victim shared kinship with the actor.
Third-party punishment may have developed as a way to maintain control in extremely large human societies. Chimp societies generally have fewer than a hundred individuals. If chimp A steals from chimp B, chances are chimp B will know about it and handle things himself. They may not have evolved a need for outsiders to involve themselves.
More on this from Ed Yong at Not Exactly Rocket Science.