What evolutionary purpose does the impulse for revenge serve?
It's got costs, but it does look like, from the best models we have, that individuals with a taste for punishing those who have harmed them could become a major part of a group. The way revenge seems to operate in our minds today really does have a functional ring to it.
The loudest way to exact revenge is to make a person's gains less profitable. You have reached into their accounting system and changed what they've gained from harming you.
The interesting thing is that the desire for revenge goes up if there are people who have watched you be mistreated, because in that case, the costs have gotten bigger. If you don't take revenge, there's a chance that people will learn that you are the type of person who will put up with mistreatment. That is the kind of phenomenon that you would expect if there is a functional logic underlying the system that produces revenge. This is a well-tuned system that's highly specific in what it cares about and the kinds of responses that it generates.
If it's so well tuned in humans, do we see this sort of behavior in other animals?
Absolutely. Imposing costs on individuals that have imposed costs on you is really common in nonhuman animals. We see it in birds. We see it in fish. It does actually seem to change them. It produces reformed behavior—the way it ought to if it's designed for deterrence.
Does the killing of bin Laden qualify as revenge?
Maybe. These things quickly get out of control once they move beyond individuals acting on their own behalf to a state level. But I think the logic is still there. One of the goals was to change our enemy's incentives for continuing the violent struggle against us. In so far as the goal is to signal a message—to change people's incentive to harm us, to have them say, "This is not leading us where we thought it was"—if you've reached into their heads, then you've effected revenge.
Does an action have to be effective, then, to be considered revenge?
It doesn't have to be effective, and that's really important. When someone gets treated kind of shoddily when driving his or her car, and that driver chooses to blare the horn or make a finger gesture, it's unlikely that the other driver is going to say, "Gee, I'm not going to cut that guy off again," because if you're driving on I-95, you're probably never going to see that driver again. We're not going to modify that individual’s calculation of what's gained by harming us personally because that's not the world we live in anymore.
It seems senseless in a modern world. But in the world in which our minds were fine-tuned—when there were fewer people and smaller groups—it would have been effective at changing people's minds.
We have learned that revenge can activate pleasure centers in the brain and lead to that so-called "sweet taste of revenge." But what other effects does it have on the brain?
We don't feel revenge for just anybody. It's generally toward people who have imposed a cost on us personally or someone we care about and know well. So as an outsider watching things, you're not going to experience the same phenomenology.
One of the really interesting things is that when someone's insulted or mistreated, not only do you get activation of the brain's reward anticipation system, you also get a lot of activity in the left prefrontal cortex of the brain. This is a signature pattern of activation in the planning of a goal.
Let's say there's something you want to work toward, like training for a half marathon. When we're planning those sorts of goals, we see differential activation in the left prefrontal cortex as we figure out: How do we get toward something we really want? Revenge, then, is like a craving—"I have to get this thing I really want." It tells us something about the kind of tissue upstairs that's getting recruited to enact revenge: it's planning a multistep process to work toward a goal we really want to obtain.