The most benighted form of institutionalized violence is human sacrifice: the torture and killing of an innocent person to slake a deity's thirst for blood.

The biblical story of the binding of Isaac shows that human sacrifice was far from unthinkable in the 1st millennium BCE. The Israelites boasted that their god was morally superior to those of the neighboring tribes because he demanded only that sheep and cattle be slaughtered on his behalf, not children. But the temptation must have been around, because the Israelites saw fit to outlaw it in Leviticus 18:21: "You shall not give any of your children to devote them by fire to Molech, and so profane the name of your God." For centuries their descendants would have to take measures against people backsliding into the custom. In the 7th century BCE, King Josiah defiled the sacrificial arena of Tophet so "that no one might burn his son or his daughter as an offering to Molech." After their return from Babylon, the practice of human sacrifice died out among Jews, but it survived as an ideal in one of its breakaway sects, which believed that God accepted the torture-sacrifice of an innocent man in exchange for not visiting a worse fate on the rest of humanity. The sect is called Christianity.

Human sacrifice was more than a riveting myth. Two millennia ago the Roman historian Tacitus left eyewitness accounts of the practice among Germanic tribes. Plutarch described it taking place in Carthage, where tourists today can see the charred remains of the sacrificial children. It has been documented among traditional Hawaiians, Scandinavians, Incas, and Celts (remember Bog Man?). It was a veritable industry among the Aztecs in Mexico, the Khonds in southeast India, and the Ashanti, Benin, and Dahomey kingdoms in western Africa, where victims were sacrificed by the thousands. Matthew White estimates that between the years 1440 and 1524 CE the Aztecs sacrificed about forty people a day, 1.2 million people in all.

Human sacrifice appears in the mythology of all the major civilizations. In addition to the Hebrew and Christian Bibles, it is recounted in the Greek legend in which Agamemnon sacrifices his daughter Iphigenia in hopes of bringing a fair wind for his war fleet; in the episode in Roman history in which four slaves were buried alive to keep Hannibal at bay; in a Druid legend from Wales in which priests killed a child to stop the disappearance of building materials for a fort; and in many legends surrounding the multiarmed Hindu goddess Kali and the feathered Aztec god Quetzalcoatl.

The killing of innocents was often combined with other superstitious customs. Foundation sacrifices, in which a victim was interred in the foundation of a fort, palace, or temple to mitigate the effrontery of intruding into the gods' lofty realm, were performed in Wales, Germany, India, Japan, and China. Another bright idea that was independently discovered in many kingdoms (including Sumeria, Egypt, China, and Japan) was the burial sacrifice: when a king died, his retinue and harem would be buried with him. The Indian practice of suttee, in which a widow would join her late husband on the funeral pyre, is yet another variation. About 200,000 women suffered these pointless deaths between the Middle Ages and 1829, when the practice was outlawed.

Human sacrifice is usually preceded by torture. The Aztecs, for example, lowered their victims into a fire, lifted them out before they died, and cut the beating hearts out of their chests (a spectacle incongruously reenacted in Indiana Jones and the Temple of Doom as a sacrifice to Kali in 1930s India). The Dayaks of Borneo inflicted death by a thousand cuts, slowly bleeding the victim to death with bamboo needles and blades. To meet the demand for sacrificial victims, the Aztecs went to war to capture prisoners, and the Khonds raised them for that purpose from childhood.

What were these people thinking? Many institutionalized killings, however unforgivable, are at least understandable. People in power kill in order to eliminate enemies, deter troublemakers, or demonstrate their prowess. But sacrificing harmless children, going to war to capture victims, and raising a doomed caste from childhood hardly seem like cost-effective ways to stay in power.

In an insightful book on the history of force, the political scientist James Payne suggests that ancient peoples put a low value on other people's lives because pain and death were so common in their own. This set a low threshold for any practice that had a chance of bringing them an advantage, even if the price was the lives of others. And if the ancients believed in gods, as most people do, then human sacrifice could easily have been seen as offering them that advantage. "Their primitive world was full of dangers, suffering, and nasty surprises, including plagues, famines, and wars. It would be natural for them to ask, 'What kind of god would create such a world?' A plausible answer was: a sadistic god, a god who liked to see people bleed and suffer." So, they might think, if these gods have a minimum daily requirement of human gore, why not be proactive about it? Better him than me.

Human sacrifice was eliminated in some parts of the world by Christian proselytizers, such as Saint Patrick in Ireland, and in others by European colonial powers like the British in Africa and India. Charles Napier, the British army's commander in chief in India, faced with local complaints about the abolition of suttee, replied, "You say that it is your custom to burn widows. Very well. We also have a custom: when men burn a woman alive, we tie a rope around their necks and we hang them. Build your funeral pyre; beside it, my carpenters will build a gallows. You may follow your custom. And then we will follow ours."

In most places, though, human sacrifice died out on its own. It was abandoned by the Israelites around 600 BCE, and by the Greeks, Romans, Chinese, and Japanese a few centuries later. Something about mature, literate states eventually leads them to think the better of human sacrifice. One possibility is that the combination of a literate elite, the rudiments of historical scholarship, and contacts with neighboring societies gives people the means to figure out that the bloodthirsty-god hypothesis is incorrect. They infer that throwing a virgin into a volcano does not, in fact, cure diseases, defeat enemies, or bring them good weather. Another possibility, favored by Payne, is that a more affluent and predictable life erodes people's fatalism and elevates their valuation of other people's lives. Both theories are plausible, but neither is easy to prove, because it's hard to find any scientific or economic advance that coincides with the abandonment of human sacrifice.

A sanguinary god that hungers for indiscriminate human scapegoats is a rather crude theory of misfortune. When people outgrow it, they are still apt to look to supernatural explanations for bad things that happen to them. The difference is that their explanations become more finely tuned to their particulars. They still feel they have been targeted by supernatural forces, but the forces are wielded by a specific individual rather than a generic god. The name for such an individual is a witch.

The transition away from human sacrifice always has a moral coloring. The people who live through the abolition know they have made progress, and they look with disgust at the unenlightened foreigners who cling to the old ways. One episode in Japan illustrates the expansion of sympathy that must contribute to abolition. When the emperor's brother died in 2 BCE, his entourage was buried with him in a traditional funeral sacrifice. But the victims didn't die for several days, and they "wept and wailed at night," upsetting the emperor and other witnesses. When the emperor's wife died five years later, he changed the custom so that clay images were placed in the tomb instead of live humans. As Payne notes, "The emperor shortchanged the gods because spending human lives had become too dear."

Witchcraft is one of the most common motives for revenge among hunter-gatherer and tribal societies. In their theory of causation, there is no such thing as a natural death. Any fatality that cannot be explained by an observable cause Is explained by an unobservable one, namely sorcery. It seems incredible to us that so many societies have sanctioned cold-blooded murder for screwball reasons. But certain features of human cognition, combined with certain recurring conflicts of interest, make it a bit more comprehensible. The brain has evolved to ferret out hidden powers in nature, including those that no one can see. Once you start rummaging around in the realm of the unverifiable there is considerable room for creativity, and accusations of sorcery are often blended with self-serving motives. Tribal people, anthropologists have shown, often single out despised in-laws for allegations of witchcraft, a convenient pretext to have them executed. The accusations may also be used to cut a rival down to size (especially one who has boasted that he really does have magical powers), to claim to be holier than everyone else when competing in the local reputational sweepstakes, or to dispose of ornery, eccentric, or burdensome neighbors, especially ones who have no supporting relatives to avenge their deaths.

Moralistic accusations can sometimes escalate into denunciations of those who fail to make moralistic accusations, snowballing into extraordinary popular delusions and the madness of crowds. In the 15th century two monks published an exposé of witches called Malleus Maleficarum, which the historian Anthony Grafton has called "a strange amalgam of Monty Python and Mein Kampf." Egged on by its revelations, and inspired by the injunction in Exodus 22:18 "Thou shalt not suffer a witch to live," French and German witch-hunters killed between 60,000 and 100,000 accused witches (85 percent of them women) during the next two centuries. The executions, usually by burning at the stake, followed an ordeal of torture in which the women confessed to such crimes as eating babies, wrecking ships, destroying crops, flying on broomsticks on the Sabbath, copulating with devils, transforming their demon lovers into cats and dogs, and making ordinary men impotent by convincing them that they had lost their penises.

People may also use allegations of witchcraft to recoup some of the losses from a misfortune by holding another party liable -- a bit like American accident victims who trip on a crack or spill hot coffee on themselves and sue everyone in sight. And perhaps the most potent motive is to deter adversaries from plotting against them and covering their tracks: the plotters may be able to disprove any physical connection to the attack, but they can never disprove a nonphysical connection. In Mario Puzo's novel The Godfather, Vito Corleone is credited with the principle "Accidents don't happen to people who take accidents as a personal insult." In the movie version, he spells it out to the heads of the other crime families: "I'm a superstitious man. And if some unlucky accident should befall my son, if my son is struck by a bolt of lightning, I will blame some of the people here."

Witch hunts are always vulnerable to common sense. Objectively speaking, it is impossible for a woman to fly on a broomstick or to turn a man into a cat, and these facts are not too hard to demonstrate if enough people are allowed to compare notes and question popular beliefs. Throughout the Middle Ages there were scattered clerics and politicians who pointed out the obvious, namely that there is no such thing as a witch, and so persecuting someone for witchcraft was a moral abomination. (Unfortunately, some of these skeptics ended up in the torture chambers themselves.) These voices became more prominent during the Age of Reason, and included influential writers such as Erasmus, Montaigne, and Hobbes.

The psychology of witchcraft accusations can shade into other blood libels, such as the recurring rumors in medieval Europe that Jews poisoned the wells or killed Christian children during Passover to use their blood for matzo. Thousands of Jews were massacred in England, France, Germany, and the Low Countries during the Middle Ages, emptying entire regions of their Jewish populations.

The Duke of Brunswick in Germany was so shocked by the methods used by Inquisitors in his duchy that he asked two famous Jesuit scholars to supervise the hearings.

Some officials became infected with the scientific spirit and tested the witchcraft hypothesis for themselves. A Milanese judge killed his mule, accused his servant of committing the misdeed, and had him subjected to torture, whereupon the man confessed to the crime; he even refused to recant on the gallows for fear of being tortured again. (Today this experiment would not be approved by committees for the protection of human subjects in research.) The judge then abolished the use of torture in his court. The writer Daniel Mannix recounts another demonstration:

The priests followed him to where a wretched woman was being stretched on the rack.

"Come with me to the torture chamber," suggested the Duke.

After a careful study the Jesuits told the Duke, "The Inquisitors are doing their duty. They are arresting only people who have been implicated by the confession of other witches."

"What else do you know about them?" demanded the Duke.

"No, no!" screamed the woman. "You are quite right. I have often seen them at the Sabbat. They can turn themselves into goats, wolves, and other animals."

"Several witches have had children by them. One woman even had eight children whom these men fathered. The children had heads like toads and legs like spiders."

"Let me question her," suggested the Duke. "Now woman, you are a confessed witch. I suspect these two men of being warlocks. What do you say? Another turn of the rack, executioners."

One of the Jesuits, Father Friedrich Spee, was so impressed that he wrote a book in 1631 that has been credited with ending witchcraft accusations in much of Germany. The persecution of witches began to subside during the 17th century, when several European states abolished it. The year 1716 was the last time a woman was hanged as a witch in England, and 1749 was the last year a woman was burned as a witch anywhere in Europe.

The Duke turned to the astonished Jesuits. "Shall I put you to the torture until you confess, my friends?"

The other pressure is harder to explain but just as forceful: an increased valuation of human life and happiness. Why are we taken aback by the experiment in which a judge tortured his servant to prove that torture was immoral, harming one to help many? It is because we sympathize with other humans, even if we don't know them, by virtue of the fact that they are human, and we parlay that sympathy into bright lines that outlaw the imposition of suffering on an identifiable human being. Even if we have not eliminated the features of human nature that tempt us to blame others for our misfortunes, we have increasingly prevented that temptation from erupting in violence. An increased valuation of the well-being of other people, we shall see, was a common thread in the abandonment of other barbaric practices during the Humanitarian Revolution.

In most of the world, institutionalized superstitious killing, whether in human sacrifice, blood libel, or witch persecution, has succumbed to two pressures. One is intellectual: the realization that some events, even those with profound personal significance, must be attributed to impersonal physical forces and raw chance rather than the designs of other conscious beings. A great principle of moral advancement, on a par with "Love thy neighbor" and "All men are created equal," is the one on the bumper sticker: "Shit happens."