Squirrel monkeys' response to inequitable outcomes indicates a behavioural convergence within the primates

Area of Science:

• Primate behavior
• Comparative psychology
• Evolutionary biology

Background:

• Negative responses to inequity are observed in some primates, but the evolutionary origin (homology vs. convergence) is unclear.
• Previous research links inequity aversion to cooperative behaviors, as all documented species showing this response are cooperative.

Purpose of the Study:

• To investigate the response to inequity in squirrel monkeys, a species that does not extensively cooperate.
• To test the hypothesis that negative inequity responses are linked to cooperation and may have evolved convergently.

Main Methods:

• Squirrel monkeys performed a token exchange task for food rewards.
• Experimental conditions varied the effort required and the reward received.
• Behavioral responses to unequal effort-reward ratios were analyzed.

Main Results:

• Squirrel monkeys did not exhibit negative responses to inequity.
• Monkeys were sensitive to reward variation, with males showing a contrast effect.
• Sensitivity to reward differences was higher within the task context compared to free rewards.

Conclusions:

• The findings support the hypothesis that negative inequity responses evolved convergently in primates.
• The absence of inequity aversion in non-cooperative squirrel monkeys strengthens this conclusion.
• Inequity aversion may function as a mechanism for evaluating partner outcomes in cooperative contexts.