Ultimatum game demonstrates chimpanzee sense of fairness

Ever play ‘the ultimatum game’? There are many variations, but basically it involves two players dividing a pot of goodies. The first person proposes how to split the pot, and the second can either accept or refuse the split. If the division is accepted, they each get the agreed upon amount, but if refused, neither player gets anything. This simple game has been used to illustrate complex economical principals. It also tells us something about psychology. Consider that if person A is given $100 to divvy up, he should be able to offer person B one penny. After all, if B refuses, he gets nothing. Isn’t a penny better than nothing? Not according to almost everyone who plays the game. If A wants to end up with anything, he has to offer B a minimum amount of the pot, usually at least 20%. Fairness is valued over absolute gain by both participants, as evidenced by the fact that many player As will offer 50%.

Researchers from Georgia State University and from Emory University found a way to make chimpanzees play the ultimatum game. One chimp (A) is offered a choice of two tokens, one of which represents an equal split of a pile of bananas and the other an unequal split. A passes the token to chimp B, who can choose whether to return it to the human experimenter. If B returns the token, the food is divided as indicated (either three bananas each or five for A and one for B).

Like humans, the chimp As chose the fair division token most of the time. When playing a version of the game called ‘the dictator game’ in which person or chimp B must accept whatever A decides, the chimp As were far less likely to choose the equitable token. This was interesting because when given the choice, no chimp Bs ever refused any offer, making the ultimatum game effectively equivalent to the dictator game. Yet, the chimp As treated the two versions completely differently. Again, this is similar to typical reactions in human populations. Apparently, the chimpanzees' sense of fairness is not unlike that of humans.

For more on animal cooperation, see my post on the elephants and parrots.



Proctor, D., Williamson, R., de Waal, F., & Brosnan, S. (2013). Chimpanzees play the ultimatum game Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1220806110.