Two recent examinations suggest that torture arises not because of individual barbarity and sadism, or even because of the presence or absence of enlightened laws, but because of social and psychological structures. The 20th century provided more avenues for such structures to flourish, these analyses suggest, which is why so much more torture took place in the last 100 years. Sociologist Christopher Einolf recently compiled a history of torture. He limited his study to cases involving conduct that everyone would agree is torture: severe physical pain inflicted by a government official on a prisoner who has not been proved guilty.

From ancient times to the modern era, Einolf found, states have largely used such torture against people "who are not full members of society." This is true whether you look at those ancient times when torture was considered perfectly moral, or modern times when it is considered repugnant. Torture, Einolf found, has been largely reserved for slaves and foreigners, and religious and ethnic minorities -- in a word, outsiders. When it has been used against citizens, torture has usually been used to unearth or punish treason. (Heresy, Einolf argues, was a special kind of treason.)

The reason the past century has seen so much torture, the sociologist argues, is that the number and intensity of conflicts have increased greatly, especially along religious and ethnic lines: More societies have to deal with more outsiders than ever before.

The nature of treason, moreover, has changed. During Nero's time, treason had to involve a plot to kill the emperor. With the dissolution of monarchies, the nature of sovereignty has changed, said Einolf at the University of Virginia.

"In the past, treason was violence against the sovereign," he said. "But when the sovereign is defined as the people or the revolution or the Islamic movement, then treason can become anything.

"The Khmer Rouge [in Cambodia] had such an extreme ideology that failing to meet your rice quota was considered treason," he said. Torturers would tell farmers: "The reason you didn't meet your target was because you are trying to undermine the revolution."

Einolf's work complements three decades of research by Stanford psychologist Philip Zimbardo, author of "The Lucifer Effect," a recent book that explores how situations -- not individual aberrations -- turn ordinary people into torturers. The book includes a lengthy description of the Stanford Prison Experiment, in which Zimbardo showed that ordinary students randomly assigned to play the roles of guards employed cruel and sadistic techniques to control other volunteers who had randomly been picked to be "prisoners."

"The Stanford prison study shows, in these situations, the individual is compromised by the social situation," Zimbardo said. "He or she would do things he would never do under ordinary circumstances . . . anyone can become a torturer."

Zimbardo has helped to defend some of the U.S. Army reservists who tortured Iraqis at Abu Ghraib prison in Iraq. In "The Lucifer Effect," he quotes then-Cpl. Charles A. Graner Jr., who was responsible for committing many horrific acts, explaining his reasons for torturing Iraqi prisoners to another reservist: