Monkeys reject unequal pay

Last friday my group had a presentation on a paper by S.F.Brosnan and B.M. de Waal called Monkeys reject unequal pay. We divided the paper into 5 parts so each of us have a part in writing up a power point slides and also we all took turns when presenting in front of the class.

In evolution of cooperation it seems critical that individuals are comparing their own efforts and payoffs with those of others. When individual`s expectations are violated negative reactions may occur. This aversion to inequity is not unique only to human.In the paper authors look at the behaviour of capuchin monkeys.

The study in the paper concerns only five female monkeys which were subjected to 4 contitions. First, equality in which two monkeys exchanged tokens with human experimenter to receive cucumber. Second, inequality in which one monkey exchanged for cucumber and its partner for grape (more favoured food).Third, effort controls in which a grape was simply given to the partner by the experimenter followed by the subject herself exchanging for cucumber. Fourth, food controls in which in the absence of a partner the subject witnessed a grape being placed in the location where the partner normaly sat after which subject exchanged for cucumber.

Despite the small number of subjects ,what they found was that the strongest increase in refusal to exchange occured when a partner received better rewards without any effort.

Also authors wanted to test if behaviour would change over the course of test. Each subject received two tests of each condition, each with 25 trials.One prediction said that failed exchanges might increase over trials if subject did not recognize that they were getting lesser reward but learned it over time. The other prediction said that failed exchanges might decrease if subject settled for the lesser reward seeing that higher rewards wasn`t coming. To test those two predictions they compared subject`s responses with first 15 trials versus last 10 trials.
For equality, inequality and effort controls conditions non exchanges increased over the course of tests. For food controls condition non exchanges decreased. Maybe it was because in an absence of a partner the adjustment to low value reward might be made easier.

Capuchin monkeys seem to measure reward in similar way as people judge fairness. They compared their own rewards with those available and their own efforts with those of others.
Although the data gathered here cannot explain those responses , there is a possibility that monkeys similarly to humans are guided by social emotions.

I found this article very interesting and I am looking forward to reading more articles about aversion to inequity in animals for our end of semester work.