It used to be easy to separate man from beast. Then we realised animals, too, can experience sophisticated emotions and communicate through language. But there is one thing that is beyond even our closest relatives, chimpanzees. And that is the ability to be spiteful.
Keith Jensen and his team at the Max Planck Institute for Evolutionary Anthropology at Leipzig, Germany, conducted experiments in which they placed a food-laden table in front of a caged chimp.
Attached to the table was a string the chimp could pull to collapse the table. The chimp resisted the urge to pull the string as long as the food was within its reach. But when the researchers moved the food to the opposite side of the table, the frustrated chimp collapsed the table in 30% of the trials.
In a second experiment, the researchers placed a second chimp in a cage at the opposite side of the table. Moving the food across the table now benefited the second chimp at the cost of the first. If the first chimp wanted to be spiteful, it could simply collapse the table and prevent its rival from feeding. But the chimps tested merely showed the same level of frustration as before, collapsing the table 30% of the time.
But if the second chimp attempted to move the food closer to itself, by pulling a string of its own, the first chimp reacted angrily, collapsing the table 50% of the time.
Spite is a common human reaction, says Jensen. "Imagine you're a kid at a birthday party. The mother gives you cake, then takes it away and gives it to another kid. It's not his fault, but you'll still be annoyed with him because of his good fortune. But chimps don't care who's got the cake, just who took it from them," he explains. In other words, chimps fail to see things from another's point of view.
And if a chimp's lack of empathy leaves it unable to feel spite, it may also fail to behave altruistically, says behavioural ecologist Rufus Johnstone of the University of Cambridge in the UK. "There have been experiments that gave chimps the chance to be nice to another at no cost to themselves, but they weren't interested. They didn't have a human propensity to be nice," he says.
"This is where things get tricky," admits Jensen. "Other papers coming out of our research group show chimps are altruistic. One interpretation is that one set of researchers isn't doing their job properly, but we don't like that one! Maybe altruistic tendencies operate in a narrow range in chimps, and a broader range in humans."
In showing that chimps lack spite, the researchers may actually have shown that a set of connected emotions remains unique to humans. "Many differences obviously remain," says Jensen. "Humans actually care about outcomes affecting others. The good side of that is altruism. Spite is the evil twin that can't be separated from it."