Between two players, fairness, to the rational person, is seen is a 50:50 split. To illustrate an example let’s say there’s Kenny and Roger and these two players are to split $100 between themselves. Kenny has the task of proposing an amount and Roger can either accept or reject his offer. Now let’s say Kenny initially proposes a 90:10 split. Obviously, Roger will reject the offer since he’s aware of the unbalance of the situation and would prefer a sum of $0 rather than see Kenny get $90. The only instance in which Roger will accept Kenny’s proposal is if there is a fair distribution of money between them (i.e., 50:50). This example illustrates the Ultimatum Game.

Evolution and the Ultimatum Game, presents evidence of this same fair behavior among other animals. Two studies have shown that chimpanzees and monkeys display similar behavior.

In one study, chimpanzees were given raisins to split among each other. As in the Kenny and Roger example, one chimpanzee would make the split and the other would accept or reject the offer. Results showed that given any positive split, the chimpanzees would not reject an offer. However, the only time a chimpanzee would reject was when the responder was not given any raisins.

In a more detailed study found on the National Geographic site, Monkeys Show Sense Of Fairness, Study Says, researchers at Emory University used the concept of the Ultimatum Game on capuchin monkeys, a highly social species.

These monkeys were trained to retrieve a pebble and in return would be awarded a cucumber. Once the monkeys equated a pebble with a cucumber, the researchers paired them in teams. On occasion, after retrieving a pebble one monkey would receive a cucumber while the partner received a grape. Later on, some monkeys would receive a cucumber or grape for doing nothing at all. The continuation of these ‘unfair’ trades led to visible discontent in those monkeys who were continually given the short end of the stick. The results lead us to believe that these capuchin monkeys exhibit the notion of fairness and thusly follow the Ultimatum Game.