Take bees. You always think of the hive as the big social collective, everybody does what they’re supposed to do. But that’s not true. Workers often try to lay eggs, even though only the queen is supposed to lay eggs. If workers lay eggs, there are other workers that run around, eat the eggs, then punish the workers that laid the eggs. Wherever you find cooperation, you’ll also find punishment. Think of your own body. Each cell has its own self-interest to multiply. Why don’t they go berserk? How do you get cells to cooperate? The answer is, you punish cells that don’t cooperate. As far as we know, there is no other vertebrate species that punishes. Humans are by far the most social vertebrate species and we argue that that’s why humans are so cooperative.

How do you define altruism? In your work, you speak of reciprocity. What is that?

An act is altruistic if it benefits another at a cost to yourself, where there is no possible mechanism whereby you could gain even in the long run somehow: Long term benefit to someone else with a long term cost to yourself. We call that altruism. By the way, it could be a long-term benefit to a group at a long-term cost to yourself. We want a definition of altruism that isn’t subjective and also extends to animals. There is altruism in animals. It almost always depends on kin groups — that is, you’re nice to your kids, which is a biological objective.

There are really two types of reciprocity. Generally, before the work we did, reciprocity meant I help you if you help me. And that, of course, is not altruism. What we study — “strong reciprocity” — is a predisposition to cooperate, even when it’s costly, and a predisposition to punish violators, free riders.

The problem with the term “altruism” is that there are many forms of altruism. For instance, unconditional altruism is where I help others no matter what. I just help. That’s altruism, but it’s not strong reciprocity. Mostly people think altruism is goody-goody or warm and fuzzy. But, the biggest part of making society work is needing to retaliate, wanting to hurt people who hurt you. It’s much more important than the precondition to cooperate, because if you don’t have punishment, you can’t get cooperation. Strong reciprocity can be cooperation and conditional punishment.