A monkey in the test chamber returns a token to the experimenter with her right hand while steadying the human hand with her left hand. Her partner looks on. This is a hidden-reward exchange, i.e., the monkey does not see the reward she is to receive before successful exchange.
Credit: Gwen Bragg, Frans de Waal, PNAS/National Academy of Sciences
Like children who complain "no fair," capuchin monkeys throw fits when their companions get better treats.
In a new study, envy reared its ugly head if capuchins, primates like us, landed slices of cucumber while their cage mates received tasty grapes—considered more desirable.
The recognition of an unfair situation could be critical for maintaining relationships in cooperative societies such as those of capuchins, as well as among humans, the researchers said. The study also suggests the roots of human fairness stretch well back in evolutionary time.
"In a cooperative species, being able to distinguish when one is being treated inequitably is very useful for determining whether or not to continue cooperating with a partner," said psychologist Sarah Brosnan at Georgia State University.
Brosnan, along with lead author Megan van Wolkenten and Frans B. M. de Waal, both at Emory University in Georgia, trained 13 tufted capuchin monkeys (Cebus apella) at Emory's Yerkes National Primate Research Center to play a no-fair game. In the game, each of a pair of monkeys would hand a small granite rock to a human in exchange for a reward, either a cucumber slice or the more preferable grape.
When both monkeys received cucumber rewards, all was fine in primate land. But when one monkey handed over the granite stone and landed a grape, while monkey number two got a cucumber, madness ensued.
"They would literally take the cucumber from me and then drop it on the ground or throw it on the ground, or when I offered it to them they would simply turn around and refuse to accept it," Brosnan told LiveScience.
Further experiments ruled out greed or frustration as forces driving the capuchin monkeys to react negatively to a cucumber reward.
Primate sense of fairness
The results have implications for the evolution of fairness in humans, the authors write this week in the journal Proceedings of the National Academy of Sciences.
Capuchins are considered New World monkeys and so they are more distantly related to humans than chimpanzees. By studying fairness in primates from different evolutionary spots on the tree of life, Brosnan hopes to tease out where certain features of this trait showed up and whether humans' sense of fairness is unique to us.
The latest findings suggest that a sense of fairness is deeply ingrained in human evolutionary history rather than the idea that it's a more cultural response, and thus, learned from other humans.
"This work resonates with a lot of people, because I think all of us have had those experiences where something seems good enough until we found out that someone had a higher salary or a better start-up package," Brosnan said.
The researchers noted one difference, however, between the human and capuchin senses of fairness: While humans regard fairness as equal treatment of themselves and others, the capuchin monkeys only care about number one.
"The capuchins' sense of inequity seems to be very one-sided. It's all about whether or not 'I' got treated unfairly," Brosnan said. "That certainly implies that this stage evolved first and that may be where humans have taken the extra step and become more focused on both sides of the equation."