  The fact that humans cooperate with non-kin in large groups, or with people
they will never meet again, is a long-standing evolutionary puzzle with
profound implications. Cooperation is linked to altruism, the capacity to
perform costly acts that confer benefits on others. Theoretical approaches had
so far disregarded costly acts that do not yield future benefits for the
altruist, either directly or indirectly. Recently, strong reciprocity, i.e.,
the predisposition to cooperate with others and to punish non-cooperators at
personal cost, has been proposed as a schema for understanding altruism in
humans. While behavioral experiments support the existence of strong
reciprocity, its evolutionary origins remain unclear: group and cultural
selection are generally invoked to compensate for the negative effects that
reciprocity is assumed to have on individuals. Here we show, by means of an
agent-based model inspired on the Ultimatum Game, that selection acting on
individuals capable of other-regarding behavior can give rise to strong
reciprocity. The results, consistent with the existence of neural correlates of
fairness, are in good agreement with observations on humans and monkeys.
