Here's a tip for lawbreakers: If you ever find yourself in court, ask for a jury of chimpanzees. They'll never convict.
Despite being one of the closest living relatives to humans, chimps lack the urge to punish thieves who are caught red-handed, unless they themselves are the victims, according to a study published this week in the Proceedings of the National Academy of Sciences.
In a series of experiments involving 13 furry subjects with names like Frodo, Natascha and Ulla, the animals showed no interest in intervening when they observed a fellow chimpanzee purloining grapes and food pellets from a third chimp.
It was only when a chimp had their own treats stolen that they got angry and took action - in this case, by opening a trap door on the miscreant.
The study, according to lead author Katrin Riedl, a developmental psychologist with the Max Planck Institute of Evolutionary Anthropology, in Leipzig, Germany, suggests that the practice of punishing thievery and crimes committed against others is a uniquely human trait.
"Punishment can help maintain co-operation by deterring freeriding and cheating," wrote the study authors. "Of particular importance in large-scale human societies is third-party punishment in which individuals punish a transgressor or norm violator even when they themselves are not affected."
While similar experiments have been done with humans, this was the first time they were performed with chimpanzees, according to the study authors. Scientists noted that dominant chimps sometimes break up fights between smaller combatants and exhibit other behaviours that suggest a sensitivity to fairness, so it was possible they shared the human trait of third-party punishment.
In order to see whether this was the case, scientists constructed an elaborate, compartmental test cage. Chimpanzees could see each other through screened dividers, but could not touch each other.
In the experiment, a "victim" chimp had to remove a series of transparent dividers to manoeuvre a box of food within reaching distance. Once they were poised to grab the food, however, a second "thief" chimp was allowed to pull the box of food away with a rope.
A third chimp, called the "actor," was taught how to open a trap door beneath the food, so that it would disappear as the thief tried to eat it. If a chimp pressed the button, it was considered punishment of the thief.
The observer chimps rarely trigged the trap door, however, even if the victim was a relative.
The trap door got a lot more use when the victim chimp was allowed to trigger it. Dominant chimps were most likely to inflict punishment, while submissive chimps were far more hesitant to trigger the trap door on dominant thieves, the study found.
"Overall, chimpanzee punishment appears confined to retaliation against personal harm when the punisher is in a position of dominance," wrote the study authors. "Chimpanzee punishment is of the 'might makes right' variety."
