Long-tailed macaques: an unfairness model for humans

Area of Science:

• Primate behavior
• Comparative psychology
• Neuroeconomics

Background:

• Human primates exhibit equity aversion, avoiding unfair outcomes.
• Long-tailed macaques (Macaca fascicularis) also show aversion to inequity.
• Food preference influences decision-making in non-human primates.

Purpose of the Study:

• To investigate the drivers of unfair behavior (equity aversion) in long-tailed macaques.
• To determine if macaques exhibit equity preference.
• To assess the role of food type preference in aversion to distribution ratios.

Main Methods:

• A food preference test identified preferred outcomes (red grapes over vanilla wafers).
• Experiments involved distributing grapes at 1:1 (equity) and 0:2, 1:3 (inequity) ratios among kin-pairs.
• Aversion (Av) versus acceptance (Ac) responses were recorded over 60 trials per condition.

Main Results:

• Aversion to inequity (1:3 ratio) was not significantly different from aversion to equity (1:1 ratio).
• Macaca fascicularis showed aversion even in equitable 1:1 distributions.
• Observed aversion correlated with subjective food preferences, not distribution fairness.

Conclusions:

• Equity aversion in long-tailed macaques is primarily driven by outcome preference, not distribution ratios.
• Subjective food preferences can lead to seemingly irrational behavior, like rejecting equal food sharing.
• This suggests a complex interplay between preference and fairness in primate decision-making.