Does inequity aversion depend on a frustration effect? A test with capuchin monkeys (Cebus apella)

Area of Science:

• Primate behavior
• Comparative psychology
• Animal cognition

Background:

• Previous studies suggested inequity aversion in primates, where observing a conspecific receive a better reward led to rejection of lower rewards.
• Later research proposed that frustration, caused by switching roles between model and witness, might explain these observed effects.

Purpose of the Study:

• To investigate whether inequity aversion in capuchin monkeys persists when the frustration-inducing element of role-switching is removed.
• To determine if observed inequity effects are genuinely driven by aversion to unfairness or by frustration.

Main Methods:

• Pairs of female capuchins participated in a token exchange task.
• Witness monkeys received a low-value food regardless of the model's reward, eliminating role-switching.
• Equity and inequity conditions were established based on the adjacent model's food reward (same or higher value).

Main Results:

• Witness capuchins showed no difference in their willingness or speed to exchange tokens for a low-value food, regardless of whether the adjacent model received a high- or low-value food.
• Models were also unaffected by their history of food rewards, indicating no frustration.
• These results demonstrate a lack of inequity aversion in this experimental setup.

Conclusions:

• The study provides evidence against inequity aversion in capuchins when frustration-induction procedures are controlled.
• Observed inequity effects in nonhuman primates may be artifacts of experimental design, specifically frustration induced by role-switching.
• The labile nature of frustration induction in primates could explain the inconsistent emergence of inequity effects in prior research.