 Three-move games with a slightly different structure have received attention under the label “optional PD.” See, for example, Kitcher (2011), Kitcher (1993), Batali and Kitcher, Szabó and Hauert, Orbell and Dawes (1993), and Orbell and Dawes (1991). The first three sources take optional games also to allow players to signal willingness to engage (i.e., play \(\bC\) or \(\bD\) against) particular opponents. The simple three-move games without signaling discussed in this section are called “semi-optional” in Batali and Kitcher. \(S,R,P\) and \(T\) payoffs are ordered as before, but the payoff matrix now contains, in addition, an “opt-out” value, \(O\), that lies between \(P\) and \(R\).