 Nowak and May have investigated in greater detail SPDs in which the only permitted strategies are \(\bCu\) and \(\bDu\). (These are the strategies appropriate among individuals lacking memory or recognition skills.) They find that, for a variety of spatial configurations and distributions of strategies, evolution depends on relative payoffs in a uniform way. When the temptation payoff is sufficiently high, clusters of \(\bDu\) grow and those of \(\bCu\) shrink; when it is sufficiently low, the \(\bDu\) clusters shrink and the \(\bCu\) clusters grow. For a narrow range of intermediate values, we get successions of complex patterns like those noted by Axelrod. The evolving patterns exhibit great variety. For a given spatial configuration, however, the ratio \(\bCu\) to \(\bDu\) strategies seems to approach the same constant value for all initial distributions of strategies and all temptation payoffs within the special range. The idea that these simulations partially explain the persistence of cooperation in nature has been questioned on the grounds that they assume deterministic (error-free) moves and updates. But the authors report similar phenomena under a variety of error-conditions, although lower relative temptation values are then required for the survival of the cooperators, and the level of error cannot exceed a certain threshold. (See Mukherjii et al, and the reply by Nowak et al that immediately follows it.)