In an unintentionally hilarious video clip, primatologist Frans de Waal narrates an experiment conducted in his laboratory at Emory University involving capuchin monkeys. One monkey exchanges a rock for a cucumber slice, which he gleefully ingests. But after seeing another monkey receive a much tastier grape for a rock, he angrily hurls it back at the experimenter when he is again offered a cucumber slice. He rattles the cage wall, slaps the floor and looks seriously peeved at this blatant injustice. (See the video at

A sense of justice and injustice—right and wrong—is an evolved moral emotion to signal to others that if exchanges are not fair there will be a price to pay. How high a price? In the Ultimatum Game, in which one person is given a sum of money to divide with another person—with the stipulation that if the offer is accepted both keep the money, but if the offer is rejected no one gets any money—offers less than 30 percent of the sum are typically rejected. That is, we are willing to pay 30 percent to punish an offender. This is called moralistic punishment.

In a classic 1983 article entitled “Crime as Social Control,” sociologist Donald Black, now at the University of Virginia, notes that only about 10 percent of homicides are predatory in nature—murders that occur during a burglary or robbery. The other 90 percent are moralistic, a form of capital punishment in which the perpetrators are the judge, jury and executioner of a victim they perceive to have wronged them in some manner deserving of the death penalty. Black's disturbing examples include a man who “killed his wife after she ‘dared’ him to do so during an argument,” a woman who “killed her husband during a quarrel in which the man struck her daughter,” a man who “killed his brother during a heated discussion about the latter's sexual advances toward his younger sisters,” a woman who “killed her 21-year-old son because he had been ‘fooling around with homosexuals and drugs,’” and others “during altercations over the parking of an automobile.” Recall the murder of three Muslims in Chapel Hill, N.C., this past February, which at least partly involved a parking spot dispute.

After the Middle Ages, such morally motivated self-help justice was replaced for the most part by rationally motivated criminal justice. Black notes, however, that when people do not trust the state's justice system or believe it to be biased against them—or when people live in weak states with corrupt governments or in effectively stateless societies—they take the law into their own hands. Terrorism is one such activity, the expression of which, Black argues in a 2004 article in Sociological Theory entitled “The Geometry of Terrorism,” is a form of self-help justice whose motives depend on the particular terrorist group. These have ranged from revolutionary Marxism in the 1970s to apocalyptic Islam today as practiced by the Islamic State of Iraq and Syria (known as ISIS or ISIL), which is not a state at all but a loose confederation of jihadists.

Many American liberals and media pundits have downplayed their religious motives, but as Black told me in an e-mail, “Muslim terrorists should be taken at their word that their movement is Islamic, anti-Christian, anti-Jewish, etc. We have their word as evidence, and in my view that is the proper basis on which to classify their movement. Would we have said that the violence used by Protestants and Catholics during the Protestant Reformation had nothing to do with religion? That would be absurd.”

No less absurd is the belief that jihadists are secular political agitators in religious cloak. As Graeme Wood writes in “What ISIS Really Wants,” his investigative piece in the March issue of the Atlantic, “much of what the group does looks nonsensical except in light of a sincere, carefully considered commitment to returning civilization to a seventh-century legal environment, and ultimately to bringing about the apocalypse.” Yes, ISIS has attracted the disaffected from around the world, but “the religion preached by its most ardent followers derives from coherent and even learned interpretations of Islam,” Wood concludes, adding that its theology “must be understood to be combatted.”