Cooperation and the common enemy effect

This paper presents a game-theoretic rationale for the beneficial effect of a
common enemy on cooperation. In a defence game against a common
natural threat, the value of the public good of defence is equal to the sum of
the players’ defensive efforts. The game therefore takes the form of a
prisoner’s dilemma, leading to free-riding. When the same defence game is
played against a common enemy, the value of the public good of defence is
equal to the smallest defensive effort. The game now takes the form of a
stag hunt, so that a cooperative equilibrium becomes possible. For this
reason, an informed and benevolent government may not want to inform
the public that it is facing a common natural threat rather than a common
enemy. At the same time, the common enemy has an incentive to mimic
nature, and perform only random rather than targeted attacks.