In the first experimental demonstration of its kind,
researchers led by Sarah Brosnan and Frans de Waal at the Yerkes
National Primate Research Center and the Living Links Center have
shown nonhuman primates respond negatively to unequal reward distribution,
a reaction often seen in humans based on their universal sense of
While researchers long have recognized the sense of fairness within
the human species, Brosnan and de Waal are the first to confirm
this trait in nonhuman primates. The findings appeared in the Sept.
18 issue of Nature and have been reported in several national
These new findings, coupled with previous scientific data that demonstrate
a direct link between nonhuman primate behavior and that of humans,
support a new school of thought that economic decision-making is
based as much on an emotional sense of fairness as on rational considerations.
Identifying similar reactions in nonhuman primates as in humans
offers insight into how such emotional reactions developed, providing
researchers and economists with a new perspective on why humans
make certain economic decisions in relation to efforts, gains and
losses of others.
People often forgo an available reward because it is not what
they expect or think is fair, Brosnan said. Such irrational
behavior has baffled scientists and economists, who traditionally
have argued all economic decisions are rational. Our findings in
nonhuman primates indicate the emotional sense of fairness plays
a key role in such decision-making.
Brosnan and de Waal conducted four tests, each including two sessions
of 25 trials, on pairs of female brown capuchin monkeys. First they
gave study subjects lower-value rewards of cucumbers if the subjects
would exchange tokens. Then, they measured the study subjects
responses when grapes, a higher-value reward, were given to their
partners for exerting varying levels of work.
The subjects compared their rewards with those of their partners
and refused to accept a lower-value reward if their partners received
a higher-value reward, says Brosnan, This effect is
amplified when the partner does not have to work for the reward.
The researchers recorded a 95 percent completed exchange rate with
the subjects during the equity test, in which both subject and partner
received cucumber as the reward for the same amount of work. The
completed exchange rate fell to 60 percent during the inequity test,
in which subjects observed their partners receiving grapes for completing
the same amount of work. A further decrease to 20 percent of completed
exchanges occurred in the effort-control test, when partners received
the higher-value reward for less work. Finally, a 55 percent exchange
rate was recorded for the cucumbers in the food-control test.
Brosnan and de Waal are conducting related studies in capuchins
to further explain these responses. They also are conducting a similar
study with chimpanzees.
The goal of the Living Links Center at the Yerkes National Primate
Research Center is to view great apes as a window to the human past
by studying their behavior, cognition, neuroanatomy, genes and reproduction
in a noninvasive manner. Another goal is to educate the public about
apes and to help guarantee their continued existence in the wild.