You know what happens when you play the prisoner’s dilemma? Everyone loses. After all, we’re self-interested little buggers. It’s a Hobbes eat Hobbes sort of world.
Or is it? In real, one shot games, people cooperate in surprising numbers: more than a third of the time. Even when they’ve been told that the other person has cooperated, and they can thereby stiff them by defecting, people still cooperate. In fact, knowing that the other person has already cooperated actually increases rates of cooperation dramatically.
Fancy that! Cooperation, in a one shot game, with a stranger you’ll never meet again. And no gain to you from cooperating except the rosy glow of being a good egg. Well, on balance, maybe altruism pays off, even if particular episodes of altruism don’t. Wouldn’t it therefore be best to feel good about it as a general principle?