  This paper discusses the role of opportunistic punisher who may act selfishly
to free-ride cooperators or not to be exploited by defectors. To consider
opportunistic punisher, we make a change to the sequence of one-shot public
good game; instead of putting action choice first before punishment, the
commitment of punishment is declared first before choosing the action of each
participant. In this commitment-first setting, punisher may use information
about her team, and may defect to increase her fitness in the team. Reversing
sequence of public good game can induce different behavior of punisher, which
cannot be considered in standard setting where punisher always chooses
cooperation. Based on stochastic dynamics developed by evolutionary economists
and biologists, we show that opportunistic punisher can make cooperation evolve
where cooperative punisher fails. This alternative route for the evolution of
cooperation relies paradoxically on the players' selfishness to profit from
others' unconditional cooperation and defection.
