 In this version of the game, defection is no longer a dominant move and mutual defection is no longer an equilibrium outcome. If Column cooperates, Row does best by defecting; if Column defects, Row does best by playing \(\bN\); and if Column plays \(\bN\), then Row does equally well by playing any move. From the outcome of mutual \(\bD\) either player can benefit by unilaterally switching to \(\bN\). But from the outcome of mutual \(\bN\), neither party can benefit by unilaterally changing moves. So the optional PD is a weak equilibrium PD, with \(\bN\) playing the role of defection. Orbell and Dawes (1991 and 1993) add the additional condition that the opt-out payoff \(O\) is equal to zero. In an optional PD, a rational player will engage (i.e., play either \(\bC\) or \(\bD\)) if and only if she expects her opponent to cooperate. For, if her opponent does cooperate, she will be guaranteed at least \(R\) by engaging and exactly \(O\) by not engaging, whereas if her opponent does not cooperate she will be guaranteed at most \(P\) by engaging and exactly \(O\) by not engaging. This feature becomes especially salient when \(O\) is zero, for then the payoff for engaging is positive if and only if one's opponent cooperates.