 In IPD tournaments like those of Axelrod and Kendall et al, players know nothing about each other except their moves in the game, and so they can use no other information to signal their membership in a group. In the real world it would seem much more likely that other avenues of communication would be available. The notion that cooperative outcomes might be facilitated by such communication among players is an old idea in game theory. Santos et al show how this might be possible. Their work borrows from an influential paper by Arthur Robson (1990). Imagine an evolutionary game, whose underlying payoff structure may be a stag hunt or a PD, in which all players can choose only between (unconditioned) moves \(\bC\) and \(\bD\). If the underlying game is a PD the population will stablize with universal defection. (If it is stag hunt it might stabilize either with everyone playing \(\bD\) (the inferior equilibrium) or everyone playing \(\bC\) (the superior equilibrium). To illustrate the beneficial possibilities of communication, let us suppose the former.) Now suppose a small group of mutants enter the population who make a signal (the “secret handshake”) and play \(\bC\) against those who signal and \(\bD\) against all others. Since these players do as well as the originals against ousiders and better against themselves, they will soon take over the population. (And this would not be true of the other possible mutants with similar resources, like those signaling and playing \(\bD\) against themselves and \(\bC\) against outsiders.) So communications does seem to facilitate cooperation. If the underlying game is a PD, however, once the new uniform cooperating population has taken over, it is itself vulnerable. It can be invaded and supplanted by a “deceiving”) invader who signals and defects against signallers. The resulting population can then be infiltrated (but not supplanted) by other, non-signaling, defectors. So Robson concluded that signaling could move a population from the inferior equilibrium to the superior one in an evolutionary stag hunt, but could only delay the establishment of universal defection in a PD. Santos et al observe, however, that, if a second signal is available, the universally defecting population could be supplanted by a small group of mutants using it as new secret handshake. Of course this group is itself vulnerable to mutants who mimic the second signal while defecting against all. In this case however, the resulting dark age may no longer be permanent. If a mutant group of signal-one handshakers re-emerges before any signal-one defectors have drifted into the population, they will again take over, and the cycle will be repeated. (If a third signal were available, of course, the return of cooperation would be even easier.) The story is no longer one where “evolution stops”: rather the population cycles between states of universal defection and universal cooperation. The time spent in each state depends on the payoffs of the PD and the number of available signals. Santos et al demonstrate, however, that, for finite populations with sufficiently slow mutation rates and large numbers of available signals cooperation predominates in EPDs with signaling. 