What’s going on in their heads? (Photo by John Moore/Getty Images)

Of all the characters in the Senate’s report on CIA’s harsh interrogation techniques — which many refer to as “torture” — we know the least about those who performed them.

The political class is known; the only question is what they knew about the interrogation methods — including waterboarding, and lengthy forced periods without sleep — that CIA employed at secret prisons during the Bush administration.

The psychologists who developed the CIA’s interrogation program are also well-known, though they claim that the Senate committee “cherry picked things.” All of the detainees themselves are known.

But the rank and file remain anonymous.

We know that 85 percent of them were contractors, few had actual interrogation experience, and that “numerous” agents had “serious documented personal and professional problems” that “should have called into question” their continued employment by the CIA and access to classified information, according to the new Senate report. We also know that they used some of the most disturbing interrogation techniques in American history.

We know a lot about what happens to the detainees. Survivors of harsh interrogations, whether or not they meet the legal definition of torture, are willing to talk about their experiences, enabling fairly substantial research into how the experience of such physical and mental duress changes their psychology.

But what about the effect of harsh interrogation on the interrogators themselves? What can we expect for the future of those who carried out the rectal feedings, waterboardings, and other harsh treatment of detainees that the report described?

That we know less about. Most of the research that does exist focuses on cases in which people committed torture. (Whether the CIA’s techniques meet that definition is a matter of dispute.)

The Washington Post's Greg Miller lists the important takeaways from the CIA interrogation report and explains why it is being released now. (The Washington Post)

In the post World War II era, there have been only about 140 analyses or interviews of people accused of being torturers, according to Reed College professor Darius Rejali, whose research focuses on torture through the ages. They come from all over: Iran, Brazil, France, Greece, and yes, America. And Rejali has found that a few common threads stand out.

The most important thing to know: Torturers are not a few “bad apples,” predisposed by nature to cruelty. “Basically they’re normal when they go in. They’re not sadists,” Rejali says. “They’re chosen primarily because they’re loyal, they’re patriotic, and they can keep a secret.” Starting with the famous experiments of Stanley Milgram in 1963, which have been replicated since then, most experimental subjects are willing to apply pain to other people under certain conditions.

“Basically they’re normal when they go in. They’re not sadists,” Rejali says.

So how do good people end up torturing? It happens because of boredom, anger at detainees, and a kind of one-upsmanship.

“They start competing with each other for brutality,” Rejali says. “The person who breaks the other person gets all the credit.” Of course, as the Senate’s report also outlined, “breaking” prisoners rarely led to valuable intelligence, but the treatment continued.

Historically, association with torture has two primary effects, Rejali says.

First is simple burnout: Interrogation, never mind especially harsh interrogation, is hard and stressful work. That’s partly why the Army needed so many interrogators during the most intense phases of the war —1,200 were trained in 2006 alone.

And second is Post Traumatic Stress Disorder, of the sort often experienced by returning soldiers, but with higher frequency and a particularly nasty edge. The National Vietnam Veterans Readjustment Study of more than 3,000 veterans, released in 1983, found that exposure to abusive violence — including torture of Vietnamese prisoners — had the strongest correlation with the incidence of PTSD.

In democratic societies, Rejali says — more so than authoritarian regimes under which torturers were driven more by religious or ideological conviction, and thus felt comfortable with their actions — post-conflict trauma is driven by “toxic levels of guilt and shame.” Those symptoms often look similar to ones displayed by victims of torture, to the extent that refugee boards deciding whether someone should be granted asylum found it difficult to distinguish between the two.

The conflict in Iraq has yielded several anecdotal accounts of soldiers who came into contact with brutal treatment of prisoners, or engaged in it themselves. In his 2012 book, “None Of Us Were Like That Before: American Soldiers and Torture,” Joshua Phillips recounted the struggles of soldiers who had abused Iraqi detainees after they returned home. Some self-medicated with alcohol. Three in the unit Phillips followed committed suicide. Daniel Keller, who admitted to techniques like dragging prisoners through concertina wire on the floor, told Phillips he’d have no reintegration problems if he hadn’t taken part in the cruelty.

“If I hadn’t actually hurt anybody, I’d be sitting pretty—I’d be happy as could be,” Keller said.“I wouldn’t have any problems. I wouldn’t be on [expletive] medication. I wouldn’t be sitting here doing an interview because I wouldn’t know anything, and I would be [expletive] living life out there.”

In 2007, The Washington Post profiled Army interrogator Tony Lagouranis, who spoke of being tormented by the things he’d done to prisoners — like giving them hypothermia and staging executions — and feeling permanently disoriented. “It feels like fear. Of what? I’m not sure,” Lagouranis said. “You know what I think it is? You don’t know if you’ll ever regain a sense of self. How could Amy love me? I used to have a strong sense of morals. I was on the side of good. I don’t even understand the sides anymore.”

And in 2007, former contract interrogator Eric Fair wrote in The Washington Post that memories of having deprived detainees of sleep and subjected them to humiliating abuse now haunt his own dreams. “His memory harasses me as I once harassed him,” Fair wrote.

Despite those who have gone public with their experiences, much more often, the psychological ramifications of torture are magnified by the fact that they can’t talk about what happened with friends and family, or don’t want to. They’re often reluctant to confide in Veterans Administration psychologists, who could help them get psychiatric treatment, or bring them together with others who’ve had similar experiences. And it’s worse for the people at the bottom. “The people who order torture at the top of the food chain, they have lots of friends, people who tell them, ‘You did the right thing,’” Rejali says.

In their public statements after a conflict ends, Rejali says, torturers’ accounts vary in response to the context in which they’re offered: If they know they’re going to be executed, as in the case of Iranian torturers, they more often freely confess. Statements offered to truth and reconciliation commissions, however, more often tend to deny that they had a hand in the cruel behaviors of which they’re accused.

The hardest thing to deal with is the knowledge that although torture is something anyone can fall into — and, in the case of the CIA’s interrogation program, continued over the protestations of those in contact with prisoners — it’s never lionized by postwar society.

That’s why, Rejali says, military and intelligence personnel should resist the temptation to use excessive force. “Never say yes to torture,” he says. “Your country will never thank you for it.”

Leonard Bernstein contributed to this article.