In evolution, the only good is self-interested good.

But we are not restricted by evolution.

We have minds. We have memories and imaginations and culture. We can imagine the consequences of our actions and select those which suit a world we want to live in. We can enshrine the common good as a norm. We can say, “Everyone who cooperates is good and everyone who defects is evil, and evil defectors will be harshly punished.” We can say, “By cooperating for the greatest common good, we will all be elevated, so let’s do that.”

Only–

What do we do with our cooperative good when we meet someone who defects?
A neighboring village steals our crops. A friend has us paint his roof, but he is always too busy to come paint ours. A lover shares all our secrets with a gossip. A colleague takes all the credit for a shared project.

Do we hold to our ethics and keep on cooperating? Tending our crops? Painting his roof? Telling our secrets? Watch others get a promotion? Are we, in short, going to be a sucker?

Most people would agree we must retaliate. We must answer defection with defection.

So the prisoner’s dilemma is not just restricted to evolution. Even the cognitive must play it. It is a good model of any situation where what is good for the individual is not the same as what is good for the group.

Imagine another situation:

    If two villages tend their own crops, both will produce 900
    bushels of grain.

    If one village attacks the other, it could end up 1,200
    bushels of grain, and the other village will get none.
    (Whatever isn’t stolen in the attack is burned.)

    If both villages go to war against each other, each will
    produce 500 bushels of grain, since labor is devoted to
    fighting.

Both villages benefit from going to war regardless of whether the other chooses war or peace. Rationally, both must attack.