Empathy and Fairness: Psychological Mechanisms for Eliciting and Maintaining Prosociality and Cooperation in Primates

In the past, prosociality has been considered a hallmark of humans; however, recently, accumulating data have empirically
revealed that non-human animals also show prosocial behavior. In situations in which animals cannot predict return benefits,
prosocial behavior is probably driven by other-regarding motivation. A sense of fairness and empathy continue to draw attention
as the most plausible candidates for the psychological mechanisms underlying such prosocial behavior. In this article, we
first introduce comparative studies on prosocial behavior in non-human primates and discuss similarities and differences between
humans and non-human primates. Then, we discuss the role of a sense of fairness and empathy. In this paper, we hypothesize
that empathy may promote prosocial behavior, whereas a sense of fairness may play a role as a stabilizer, but not as a promoter
of prosocial behavior in non-human animals. We further hypothesize that prosocial behavior motivated by sympathetic concerns
can survive only with a sense of fairness, the inhibitory system for unnecessarily excessive expression of prosocial behavior.
Without a sense of fairness, empathic animals might be exploited by free-riders, which might lead to the extinction of cooperation.
Therefore, the interplay of a sense of fairness and empathy are both important to maintaining prosocial behavior and cooperation.
This hypothesis seems to be supported by comparative studies with non-human primates and also by neural studies with humans.