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キーワード:
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要旨:
Reciprocity and repeated games have been at the center of
attention when studying the evolution of human cooperation.
Direct reciprocity is considered to be a powerful mechanism for the
evolution of cooperation, and it is generally assumed that it can
lead to high levels of cooperation. Here we explore an openended,
infinite strategy space, where every strategy that can be
encoded by a finite state automaton is a possible mutant. Surprisingly,
we find that direct reciprocity alone does not lead to
high levels of cooperation. Instead we observe perpetual oscillations
between cooperation and defection, with defection being
substantially more frequent than cooperation. The reason for this
is that “indirect invasions” remove equilibrium strategies: every
strategy has neutral mutants, which in turn can be invaded by
other strategies. However, reciprocity is not the only way to promote
cooperation. Another mechanism for the evolution of cooperation,
which has received as much attention, is assortment
because of population structure. Here we develop a theory that
allows us to study the synergistic interaction between direct reciprocity
and assortment. This framework is particularly well suited
for understanding human interactions, which are typically repeated
and occur in relatively fluid but not unstructured populations.
We show that if repeated games are combined with only a
small amount of assortment, then natural selection favors the behavior
typically observed among humans: high levels of cooperation
implemented using conditional strategies.
