 There are at least three mechanisms by which this kind of “association” among players can be achieved. One such mechanism in evolutionary PDs has been widely studied under the label “spatial PD.” Players are arranged in some “geographical” arrangement. This may be an array with a rectangular boundary, for example, or a circle, or surface of a sphere or surface of a torus with no boundary. From the geographical arrangement two (possibly identical) kinds of neighborhoods are identified for each player. Agents meet only those in their “interaction” neighborhood, and the evolutionary dynamics considers only the payoffs to those in their “comparison” neighborhood. Generally, the evolutionary dynamics employed is one of “winner imitation” within the interaction neighborhood. (This can model either the idea that each player is invaded by its most successful neighbor or the idea that each player adopts the most successful strategy that it sees.) Because both evolution and interaction are “local,” players are more likely (after the first round) to meet those playing strategies like their own in an SPD than they would be in an ordinary evolutionary game. In addition to the “association” effects, one should also keep in mind that the outcome of SPDs may be influenced by winner imitation dynamics, which may drive to extinction strategies that might survive—and eventually predominate—with the replicator dynamics more commonly employed in ordinary EPDs.