Science
17 October 2014:
Vol. 346 no. 6207
DOI: 10.1126/science.1251776
Vol. 346 no. 6207
DOI: 10.1126/science.1251776
- Review
Evolution of responses to (un)fairness
+ Author Affiliations
- ↵*Corresponding author. E-mail: sarah.brosnan@gmail.com
Background
The human sense of fairness presents
an evolutionary puzzle because it appears to run counter to the
short-term interests
of at least some parties. We not only react
negatively to getting less than a partner but sometimes also to getting
more,
which seems illogical. Following an ideal of
impartiality, we seek appropriate outcomes for everyone within the
community,
not just a few individuals, and, in
particular, not just ourselves. Why do we react this way? Are we the
only species to do
so? Here, we consider the evidence with
regard to nonhuman primates and other animals to illuminate the
evolution of the sense
of fairness. Because social ideals escape
measurement, we focus on behavioral responses to equal versus unequal
reward division.
Moreover, a true sense of fairness includes
taking account of receiving both less than a partner and more than a
partner.
Therefore, we consider the evidence for both
of these in other species and how this informs our understanding of the
evolution
of fairness in humans.
A diagram of the relationship between first-order IA and second-order IA.
Individual A received high-level rewards, and individual B received
low-level rewards. Individuals who recognize when they
receive less than another may react
against this situation—for instance, by finding a new cooperative
partner. As reliance
on cooperation increases, individuals also
benefit from recognizing when they receive more, as this allows them to
forestall
first-order IA reactions in their partners
and thereby maintain a successful cooperative relationship. This is the
foundation
of the full-blown human sense of fairness.
A diagram of the relationship between first-order IA and second-order IA.
Individual A received high-level rewards, and individual B received
low-level rewards. Individuals who recognize when they
receive less than another may react
against this situation—for instance, by finding a new cooperative
partner. As reliance
on cooperation increases, individuals also
benefit from recognizing when they receive more, as this allows them to
forestall
first-order IA reactions in their partners
and thereby maintain a successful cooperative relationship. This is the
foundation
of the full-blown human sense of fairness.
Advances
There is widespread evidence for
sensitivity to receiving less than a partner, or “first-order inequity
aversion” (IA), in
species that cooperate outside mating bonds
and kinship. In these studies, animals are paired with a social partner
who receives
a preferred reward for completing a task.
Subjects may respond by refusing to participate or refusing to accept
the food reward,
and such reactions are compared with those
following control tests in which both subjects receive the same reward
for the
same effort. Increased responses when the
partner receives a preferred reward are indicative of a sensitivity to
inequity.
Thus far, passive and active protest
against unfavorable outcomes has been documented in monkeys, apes,
dogs, and birds. It
is thought that these species compare their
outcomes with those of others so as to judge the merit of their
partnerships.
They may turn away from partners that
appropriate more than their fair share of the yields of cooperation.
A complete sense of fairness also
includes second-order IA, however, which seeks to equalize outcomes even
at a short-term
cost to the self. This requires individuals
to give up an immediate benefit to stabilize a long-term valuable
cooperative
relationship. Second-order IA reactions have
thus far been found only in humans and apes. We hypothesize that
second-order
IA requires anticipation of first-order IA in
the partner and its negative impact on the relationship. To forestall
these
consequences, and ensure continued
cooperation, outcomes are equalized between partners.
Outlook
Thus, humans and other species seem
to share basic reactions to inequity, which serves to sustain
cooperation. We postulate
that the basic emotional reactions and
calculations underlying our sense of fairness are rooted in our primate
background
and offer a model that places these reactions
in the context of cooperative relationships.
Future research should more
explicitly investigate the key variables underlying IA, such as the
degree of dependence on cooperation,
anticipation of the way resource division
affects relationships, and the freedom to choose among and change
partners. A cross-species
investigation with a standardized paradigm,
including both first- and second-order IA, may further illuminate the
factors
involved and help verify or falsify the model
proposed.
