  Punishment and partner switching are two well-studied mechanisms that support
the evolution of cooperation. Observation of human behaviour suggests that the
extent to which punishment is adopted depends on the usage of alternative
mechanisms, including partner switching. In this study, we investigate the
combined effect of punishment and partner switching in evolutionary prisoner's
dilemma games conducted on a network. In the model, agents are located on the
network and participate in the prisoner's dilemma games with punishment. In
addition, they can opportunistically switch interaction partners to improve
their payoff. Our Monte Carlo simulation showed that a large frequency of
punishers is required to suppress defectors when the frequency of partner
switching is low. In contrast, cooperation is the most abundant strategy when
the frequency of partner switching is high regardless of the strength of
punishment. Interestingly, cooperators become abundant not because they avoid
the cost of inflicting punishment and earn a larger average payoff per game but
rather because they have more numerous opportunities to be referred as a role
agent by defectors. Our results imply that the fluidity of social relationships
has a profound effect on the adopted strategy in maintaining cooperation.
