Sarah F. Brosnan

Sarah F. Brosnan is a primatologist whose research on capuchin monkeys provides evidence that reactions to unfair treatment extend beyond human economic behavior. In an experiment with Frans B. M. de Waal, she showed that monkeys would refuse food and abandon an otherwise beneficial exchange when a partner received a better reward without a corresponding difference in effort. The study gives the book a concrete, nonhuman counterpart to human refusals of unfair offers.

The Capuchin Exchange

Brosnan and de Waal trained capuchins to hand over a granite pebble in exchange for food. When two monkeys received cucumber slices for their work, they exchanged pebble for food 95 percent of the time. The arrangement functioned smoothly so long as the rewards were alike.

The researchers then altered the terms. One monkey received a delicious grape while the other continued to receive cucumber. The less generously rewarded monkey often refused to eat the cucumber and, in 40 percent of cases, stopped trading altogether. The work required of the monkey had not changed; what changed was its view of the exchange once its partner’s better reward became visible.

Unearned Rewards and Refusal

The disparity became sharper when one monkey received a grape without doing anything. In that condition, the other monkey often threw away her pebble, and trades occurred only 20 percent of the time. The capuchin gave up a low-cost food reward rather than continue an exchange made conspicuously unequal by a partner’s unearned advantage.

The result parallels the human ultimatum game discussed in the book. Human responders routinely reject low offers, surrendering money to punish a proposer perceived as greedy; Brosnan’s monkeys likewise forgo food when a companion receives more. The comparison does not establish that capuchins reason about fairness in the same way people do, but it shows that unequal rewards can disrupt exchange even when accepting the available reward would provide an immediate benefit.

Significance in the Book

Brosnan’s study appears in the discussion of fairness and cooperation, where it challenges a narrowly self-interested account of behavior. The capuchins’ choices make the cost of unequal treatment visible: a cucumber remains edible, and pebble-for-food remains a favorable trade, yet the monkeys’ willingness to participate falls when the arrangement looks unjust. Their refusal is therefore not simply a calculation of the food’s value; it is a response to how rewards are distributed in relation to a partner’s treatment.

The experiment also extends the book’s account of social exchange beyond humans without making the monkeys mere illustrations of human behavior. Their concrete actions—rejecting cucumber, abandoning the pebble exchange, and responding especially strongly to a partner rewarded for no work—show how unequal outcomes can weaken cooperation. Brosnan’s research thus anchors the book’s treatment of fairness in observed behavior rather than in human economic theory alone.