Monkeys Reject Unequal Pay
Topic: How monkeys respond to unequal incentives
Researchers: Sarah F. Brosnan (assistant professor of Psychology, Georgia State University, US) and Frans B. M. de Waal (Professor of Psychology, Emory University, US)
Published By: Nature
Formal Reference: Sarah F. Brosnan & Frans B. M. de Waal, “Monkeys Reject Unequal Pay”, Nature 425 (2003), 297–299
Main Conclusion: Monkeys are significantly influenced by inequality in incentives relative to their peers
Description of Experiment
a. adult female capuchin monkeys participated in the experiment
b. 90% of choice tests performed, monkeys preferred grapes over cucumbers – this test refers to level of the reward: cucumber is a significantly smaller reward than a grape
c. Monkeys were requested to make an exchange with the experimenter, in couples – i.e. the two monkeys were able to see each other’s performance and rewards
d. For exchange, the monkey was given a token that could immediately be returned to the experimenter for a cucumber-slice reward. After a learning period, the exchange was successful in 95% of the cases.
e. After rewarding only one of the monkeys in each couple with grape instead of cucumber. The result was that the monkey continuing to receive cucumber moved from 95% to 50% success rate in completing the exchange.
.f. When the 2nd monkey received grapes for no effort, the 1st monkey (still rewarded by a cucumber-slice for the exchange) moved to 20% success rate of completing the exchang
Monkeys refused to participate if they witnessed their partner obtaining a more attractive reward for equal effort, an effect amplified if the partner received such a reward without any effort at all.