 Let's call this version of the game the (infinite) two-player IPD, or 2IPD. In other versions of the IPD, where pairs from a larger population come together repeatedly to play the game, a successful strategy is one that scores well. “Well” may mean (as in the case of evolution under the replicator dynamic) a score at least high as the average score in the population, or (as in the case of the evolution under the imitation dynamic) at least as high as the scores of the most successful agents in the population. Under those conditions, it is much more important, in a particular round of the game, to raise your own score than to lower your opponent's. Axelrod repeatedly (and with cause) advised participants in his tournaments not to be envious. In the 2IPD, however, the population size is two. In that case it is as valuable to lower your opponent's payoff as to raise your own, and it can even be beneficial to lower your own payoff, if doing so lowers your opponent's more than yours.