Group membership is another factor that has seen insufficient exploration in traditional Prisoner’s Dilemma analyses.  We do not encounter all six billion people on this planet with equal probability.  We usually encounter persons who are close to us (either in physical proximity or by special relationship).  Most of our encounters are with a limited group of people.  A more advanced theory of ethics must account for group membership.

There are two dimensions by which group membership can be defined:

1) One overall ethical strategy practiced by all group members.  Group members approach each other with a particular ethical strategy, a strategy different from approaching others outside the group.  Presumably, but not necessarily, the within-group strategy is more altruistic than the outsider strategy.

2) All members of a group share resources (gain) for mutual benefit.  All members contribute some of their personal gain to the group.  In turn, all members receive support from the group.

These two factors are independent of each other.  A group may share a common ethical strategy for encountering group members, yet make no provision for mutual support.  Conversely, group members may support each other, yet have no single ethical strategy for encountering other group members.

Nor need these two factors be universal among group members.  In fact, for groups with universal Good Will toward group members, there will be a strong temptation to defect (take advantage of the altruistic actions made by other group members).  In groups that provide mutual support, there will be a strong temptation toward free-riding (consume group resources without making contributions).  Groups may vary in the cohesion they exhibit in both their within-group ethical strategy and/or mutual support.

Group membership must be viewed within a larger context.  Group membership is important when a substantial proportion of encounters occur within-group.  If all encounters are with individuals outside of the group, group membership is irrelevant.  Conversely, if all encounters are with individuals within the group, then encounters with outsiders become irrelevant.  The best analysis accounts for both group and non-group encounters, balancing the proportions of the two types.