Frans B. M. de Waal

Frans B. M. de Waal appears in the book as a primatologist whose research with Sarah F. Brosnan gives concrete, animal evidence for a central claim: responses to unfairness are not confined to human economic life. In their experiment, capuchin monkeys sometimes refused food or abandoned a profitable exchange when another monkey received a better reward, making fairness visible as a force strong enough to override immediate gain.

The Experiment and Its Setup

Brosnan and de Waal trained female capuchin monkeys to exchange a granite pebble with Brosnan for food. When both monkeys received cucumber slices, the exchange proceeded readily: they traded rock for food 95 percent of the time. The arrangement established a baseline in which a modest reward was acceptable to both animals.

The researchers then disrupted that apparent equilibrium. While one monkey continued to receive cucumber, the other was given a grape, a more desirable reward. The monkey still offered cucumber often refused to eat it; in 40 percent of cases, the monkeys stopped trading altogether. The key change was not in the pebble, the work, or the cucumber, but in the unequal treatment made visible by the other monkey’s reward.

Unearned Rewards and Refusal

The imbalance became sharper when one monkey received a grape without doing anything. In that condition, the other monkey often threw away her pebble, and trades took place only 20 percent of the time. The animals thus rejected not just a less attractive payment but, in some cases, the exchange itself—even though a cucumber for a pebble had previously been accepted.

That refusal carried a cost. A cucumber remained food, and the pebble-for-food trade had been worthwhile before the comparison was introduced. Yet the monkeys’ behavior changed when a companion’s reward made the arrangement look unfair. De Waal’s experiment therefore supplies a striking counterpoint to the narrow picture of an economic actor who always takes the best available material payoff.

Fairness Beyond Human Bargaining

The book places the capuchin findings alongside the ultimatum game, in which human responders frequently reject low offers rather than let a proposer keep an unfair share. In both cases, participants surrender a benefit to register opposition to unequal treatment. The comparison does not make the animals’ situation identical to human bargaining; it shows that the impulse to resist a perceived imbalance is not presented as exclusively human.

De Waal’s contribution supports the book’s broader account of cooperation and fairness by grounding it in a specific observable choice: a monkey may discard a pebble or leave food uneaten when another receives more, especially when that other has earned nothing. The experiment turns fairness from an abstract principle into a consequence enacted at the point of exchange, where the animals’ willingness to trade collapses despite the immediate value still on offer.