So, we believe the heart of altruism is not only the willingness to cooperate and help — empathy and caring for others — but also this negative side of human nature: retaliation or retribution.

Let me give you an example that you would not even think is altruistic normally, but is: road rage.

What exactly do you mean by road rage and how is that altruistic?

Pathologically, it’s when somebody behaves badly on the road and you shoot them. Usually, though, when people drive through a yellow light or are in a wrong lane, people honk their horns, shake their fists at them. Our argument is that this behavior of getting angry at another driver, who you’re never going to see again, has strong reciprocity. It helps keep people honest.

If you don’t drive the proper way, some guy honks his horn and you feel humiliated; you’ve done a bad thing and you got caught. But he didn’t do it because he cared about keeping people honest. He honked his horn because he was pissed at you. This is true in subjective altruism. By honking your horn or yelling at someone for doing a bad thing, this is an altruistic act. It might have cost you something, not much. But it keeps the rules of the road going. It keeps people honest, so it’s an altruistic act.

You’re upholding the norm of fairness by hurting someone who was unfair. But you didn’t do it because you wanted to uphold a norm for the group. You did it because you were angry at the guy.

What about when people do something spontaneous and show genuine feelings of empathy?

Our argument is that everybody is altruistic. But for most people if the cost gets sufficiently high, they stop being altruistic.

We argue that everyday life has little bits of altruism all over the place. They’re generally not that costly, but they’re extremely important. For instance, when I go on an airplane, everyone is nice to each other; they’re never going to see each other again. Why be polite? You can imagine if you put chimps on an airplane, it would be a total disaster. Why go through these little amenities: “Can I help you with your bag?” “Let me move for you.” “Let me get up so you can get out and go to the bathroom.” Think about it. If you put a bunch of sociopaths on an airplane, it would be a disaster. But these little amenities, in everyday life, we tend to help each other even if it doesn’t cost that much. This makes society work.