In the article “Monkeys reject unequal pay”, Sarah F. Bronsnan and Frans B.M. de Waal, attempt to prove the theory that cooperative animals, such as humans and monkeys have a sense of fair play. They tested their theory using pairs of monkeys; the subjects exchanged tokens for food, whereby one food type was preferred over the second. The testing pairs of monkeys proved that like humans they too had a sense of fair play.
When they tested the pairs of monkeys a small number of them either refused to return the token or refused to eat the exchanged food. In the first part of the testing the pairs were given the same food when they exchanged their tokens but in the second testing one of the pair was given a more preferred food while the other was given a less continued to receive the same food given the previous testing. One unexpected result was the observation that the female monkeys were more reluctant to exchange food during the second testing where there was an inequality in the food reward given to the other monkey in their pairing. The data regarding the female monkeys was discarded as it was felt that there was not enough data which could prove the perceived observation that the idea of fair play was more pronounced. The testing itself proved that just like their human cousins, monkeys in general did have a sense of fair play.
The testing attempted to explain the how fair play was perceived by the higher functioning animals, such as humans and monkeys. Fair play or equality versus inequality showed the animals becoming reluctant to trade their tokens when they did not receive the same reward as their testing partner. The animal receiving the lesser treat eventually stopped accepting the reward when exchanging the token if they did not receive the better reward.
Brosnan, Sarah F. & de Wall, Frans B. M. (2003). Monkeys reject unequal. Nature, 425, 297-299