Giving is self-rewarding for monkeys

Abstract

Helping and sharing among humans is often motivated by empathy and accompanied by a sense of satisfaction. To determine whether similar self-rewarding mechanisms may underpin assistance among nonhuman primates, eight female brown capuchin monkeys (Cebus apella) underwent testing in a simple choice paradigm. Paired with a partner, subjects could select either a "selfish" option that rewarded only themselves, or a "prosocial" option that rewarded both of them. Subjects systematically favored the prosocial option provided their partner was a) familiar, b) visible, and c) receiving rewards of equal value. Prosocial tendencies increased with social closeness, being lowest toward strangers and highest toward kin. That the monkeys understood the options was suggested by greater orientation to the partner during prosocial than selfish choices. Prosocial preferences were reduced by inequity, when the partner received a superior reward. If the view between both monkeys was blocked, choices became strikingly selfish. Thus, under certain conditions, delivering benefits to others seems gratifying to nonhuman primates.

Fig. 1.

The mean (+SEM) percentage of prosocial choices across trials for tests with familiar, visible partners with equal food rewards. The 30 trials per test are divided into three blocks of 10. Chance expectation is 50%. The linear upward trend is significant (see text).

Fig. 2.

The mean (±SEM) percent prosocial choices during the third 10-trail block for all eight testing rounds in the order in which they were conducted. This graph excludes individuals with extreme (>85%) side-bias during a given round, even though these individuals were included in statistical evaluations against 50% chance (*, P < 0.05, ***, P = 0.001).

Fig. 3.

The mean (+SEM) orientation index for subject and partner during two trial phases (1, token choice; 2, reward acceptance and consumption). Data are divided according to the subject's token selection. The main significant effects (see text) were increased orientation by the subject to the other while making a prosocial choice, and increased mutual orientation during the second trial phase after prosocial choices.