Rather, I am motivated by more respectable sentiments, by compassion, love, and empathy. Not for ISIS, of course, but for their victims. I have seen the videos of decapitations and crucifixions and have read accounts of rape, slavery, and torture. If I were less invested in the suffering of their victims, I would be more receptive to a balanced discussion of different options. But because I care, I really just want them to pay.

In The Theory of Moral Sentiments, published in 1759, Adam Smith observes that when we see someone harmed by another, we feed off his desire for vengeance: “We are rejoiced to see him attack his adversary in his turn, and eager and ready to assist him.” Even if he dies, our imagination does the trick: “We enter, as it were, into his body, and in our imaginations, in some measure, animate anew the deformed and mangled carcass of the slain, [and] bring home in this manner his case to our bosoms.”

You can see this process at work in research published last year by the psychologists Anneke Buffone and Michael Poulin. Subjects were told about a competition between two students in another room of the lab. Half of the subjects read an essay in which one of the students described herself as being in distress (“I’ve never been this low on funds and it really scares me”); the others read an essay in which she was mellow (“I’ve never been this low on funds, but it doesn’t really bother me”). The subjects were then told that they were going to help out in a study of pain and performance, wherein they would get to choose how much hot sauce the student’s competitor would have to consume.

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