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.
Katrin Riedl, Keith Jensen, Josep Call, and Michael Tomasello (2012). No third-party punishment in chimpanzees PNAS DOI: 10.1073/pnas.1203179109
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