I’m not usually in favor of killing, but I’d make an exception for the leaders of ISIS. I’d feel a certain satisfaction if they were wiped off the face of the Earth. This is a pretty typical attitude, shared even by many of my more liberal friends, even though, intellectually, it’s not something that we’re comfortable with or proud of.

Where does this malice come from? Psychologists have standard explanations for murderous feelings towards groups of strangers, but none of them apply here. I don’t think ISIS is a threat to me or my family or my way of life; I’m not driven by disgust and contempt; I don’t dehumanize them; I don’t think of them as vermin or dogs.

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.

Keep in mind that this competitor didn’t do anything wrong; he or she had nothing to do with the student’s anxiety about money. Nonetheless, the subjects chose to give more hot sauce to this other person when the student was described as distressed. Their empathy drove aggression, even when it made no moral sense.

Also, before the study was done, Buffone and Poulin gave all of their subjects a test that scans for specific genes that make people more sensitive to vasopressin and oxytocin, hormones that are implicated in compassion, helping, and empathy. As predicted, there was a greater connection between empathy and aggression in those subjects that had those genes—that is, more empathic people were more aggressive when exposed to the suffering of strangers.

I’ve come up with similar findings in a series of studies done in collaboration with the Yale graduate student Nick Stagnaro. We start by giving people a simple test that measures their degree of empathy. Then we tell them some awful stories, about journalists kidnapped in the Middle East, about child abuse in the United States. And then we ask them how best to respond to those responsible for the suffering. In the Middle East case, we give a continuum of political options, from doing nothing to public criticism, all the way to a military ground invasion. For the domestic version, we ask about increased penalties for the abuser, from increasing their bail to making them eligible for the death penalty. Just as with the genetic study, we found that the more empathic people are, the more they want a harsher punishment.

Politicians are comfortable exploiting this dark side of empathy. Donald Trump likes to talk about Kate—he doesn’t use her full name, Kate Steinle, just Kate. She was murdered in San Francisco by an undocumented immigrant, and Trump wants to make her real to his audience, to make vivid his talk of Mexican killers. Similarly, Ann Coulter’s recent book, Adios, America, is rich with detailed descriptions of immigrant crimes, particularly rape and child rape, with chapter titles like “Why Do Hispanic Valedictorians Make the News, But Child Rapists Don’t?” and headings like “Lost a friend to drugs? Thank a Mexican.” Trump and Coulter use these stories to stoke our feelings for innocent victims, to motivate support for policies against the immigrants who are said to prey upon these innocents.

There is a history of this sort of thing. Lynchings in the American South were often sparked by stories of white women who were assaulted by blacks, and anti-Semitic attacks prior to the Holocaust were often motivated by tales of Jews preying on innocent German children. Who isn’t enraged by someone who hurts a child?

Similar sentiments are used to start wars. As the U.S. prepared to invade Iraq in 2003, newspapers and the Internet presented lurid tales of the abuses committed by Saddam Hussein and his sons. Israeli reaction to the news of three murdered Israeli teenagers drove public support for the recent Gaza conflict, just as Hamas used stories of murdered Palestinians to generate enthusiasm for terrorist attacks against Israel. When making the case for air strikes in Syria, Obama spoke movingly about the horrors inflicted by Assad and his soldiers, including their use of chemical warfare. Should we go to a full-scale war against ISIS, we will surely see more images of people being beheaded.

Our reaction to these atrocities can cloud our judgment, biasing us in favor of war. The benefits of war—including avenging those who have suffered—are made vivid, but the costs of war remain abstract and statistical. We see this same bias reflected in our criminal-justice system. The outrage that comes from empathy drives some of our most powerful punitive desires. It’s not an accident that so many statutes are named for dead girls—as in Megan’s Law, Jessica’s Law, and Caylee’s Law—and no surprise that there is now enthusiasm for “Kate’s Law.” The high incarceration rate in the United States, and our continued enthusiasm for the death penalty, is in part the product of fear and anger, but is also driven by the consumption of detailed stories of victims’ suffering.

Then there are victim-impact statements, where detailed descriptions of how victims are affected by a crime are used to help determine the sentence imposed on a criminal. There are arguments in favor of these statements, but given all the evidence that we are more prone to empathize with some individuals over others—with factors like race, sex, and physical attractiveness playing a powerful role—it’s hard to think of a more biased and unfair way to determine punishment.

Part of me still wishes the leaders of ISIS dead. Still, during my better moments, I acknowledge that what I really should want is for them to stop torturing and killing people, and that any violent act towards them should be judged on its probable consequences—how much it makes the world better, how it deters these sorts of acts in the future—not on how satisfying it might be to me or my friends. Everyone appreciates that fear and hate can motivate ugly choices; we should be mindful that our most tender sentiments can do the same.