Inequity responses of monkeys modified by effort

Abstract

Without joint benefits, joint actions could never have evolved. Cooperative animals need to monitor closely how large a share they receive relative to their investment toward collective goals. This work documents the sensitivity to reward division in brown, or tufted, capuchin monkeys (Cebus apella). In addition to confirming previous results with a larger subject pool, this work rules out several alternative explanations and adds data on effort sensitivity. Thirteen adult monkeys exchanged tokens for rewards, showing negative reactions to receiving a less-favored reward than their partner. Because their negative reaction could not be attributed to the mere visibility of better rewards (greed hypothesis) nor to having received such rewards in the immediate past (frustration hypothesis), it must have been caused by seeing their partner obtain the better reward. Effort had a major effect in that by far the lowest level of performance in the entire study occurred in subjects required to expend a large effort while at the same time seeing their partner receive a better reward. It is unclear whether this effort-effect was based on comparisons with the partner, but it added significantly to the intensity of the inequity response. These effects are as expected if the inequity response evolved in the context of cooperative survival strategies.

Fig. 1.

Percentage of typical exchanges in three conditions with the same level of effort. Typical exchanges were those in which subjects completed the interaction in 5 s and accepted the reward. Equality test, both receive cucumber; Equality-G, Equality test with grapes visible; Inequity test, partner receives grape, subject receives cucumber.

Fig. 2.

In the Ineq sessions, in which their partners received a better reward, subjects increased their likelihood of refusing to exchange over the course of the 25 trials. In Eq and Eq-G sessions, the subject's willingness to exchange did not change over the course of the session.

Fig. 3.

There was no evidence of a frustration effect. Subjects showed typical exchanges at about the same rate in sessions immediately after those in which they had received grapes (frustration) as in those after sessions in which they had received only cucumber (no frustration).

Fig. 4.

Individual effort magnifies the effect of inequity. Large indicates that three exchanges were required of the subject. Small indicates that only a single exchange was required. Eq indicates that both individuals received the same reward, and Ineq indicates that the partner received a better reward. (Left) Black bars show four effort conditions in which the subject received cucumber, and the partner received either cucumber also (Eq) or grape (Ineq). (Right) Gray bars show two conditions in which both individuals received grape.

Fig. 5.

A monkey in the test chamber returns a token to the experimenter with her right hand while steadying the human hand with her left hand. Her partner looks on. This is a hidden-reward exchange, i.e., the monkey does not see the reward she is to receive before successful exchange. (Drawing by Gwen Bragg and Frans de Waal after a video still.)