No doubt part of the explanation is that beheadings tend to draw more attention than suicide bombings and exploding cars. Deadly though they are, car bombs and shootings have lost much of their shock value in Western eyes. It takes an unusually high death toll for a bombing in Iraq to attract as much media attention as the decapitation of a single hostage by an English-speaking Islamist wielding a knife. Terrorists crave attention, more now in the digital age, perhaps, than ever before. Islamic State and other jihadist groups have many ways to commit mass murder. But for generating a spectacle that will be noticed — and shuddered at — the world over, sawing off the head of an American journalist or a European relief worker, then uploading the video to the Internet, is hard to beat.

Yet beheadings, too, can lose their shock value. It wasn’t all that long ago that decapitation was a familiar punishment in Western culture. As late as 1977, the guillotine was still being used for executions in France. In the colonial era, severed heads of criminals were sometimes displayed on Boston Common. Plus, modern video games and ultrarealistic computer graphics make it easier than ever to grow inured to almost any image or scene, no matter how grisly or intense or intimate.

But the beheadings aren’t just a fashion.

There is more to the Islamist passion for decapitation than psychological warfare and a hunger for notoriety. There is also Muslim theology and history, and a mandate going back to the Koran. In a 2005 study published in Middle East Quarterly, historian Timothy Furnish quotes the famous passage at Sura 47:4: “When you meet the unbelievers, smite their necks.” For centuries, Furnish observes, “leading Islamic scholars have interpreted this verse literally,” and examples abound throughout Islamic history.