 The Brown-Nash-von Neumann dynamics. One key assumption made by the learning rule which yields the replicator dynamics is that imitation is a reliable guide to future payoffs. This can be problematic for two reasons. First, the fact that a strategy has an expected payoff greater than the average payoff of the population may simply be indicative of peculiarities of the current population composition, and not of any particular strategic merit the strategy possesses. Such a learning rule may end up with much of the population shifting to adopt a strategy, only for its transient fitness benefits to disappear. Second, if a strategy is not present in the population at all, it has no chance of being adopted by imitation.