In his latest documentary “Crazy, Not Insane”—screening at documentary film festival IDFAAlex Gibney gives the floor to Dr. Dorothy Otnow Lewis, a renowned American psychiatrist who has examined numerous serial killers. Specializing in the study of people with dissociative identity disorder (DID), Lewis has concluded that many of the 20th century’s most notorious murderers—including Joel Rifkin, Joseph Paul Franklin and Arthur Shawcross—experienced horrific abuse as children. “What happens to us in our childhood can have a profound influence on who we end up as adults,” Gibney muses.

He has never been a “serial killer aficionado,” he admits, preferring to delve into the dark side of human psychology rather than wallow in grisly details. “How do you reckon with somebody who puts you on a hot radiator, burns your skin and later on gives you hugs, telling you how much they love you? Many of my films are about so-called good people who do bad things.”

Indeed, as the director tells Variety, it was actually Dr. Lewis’s dark sense of humor that attracted him to the project. “It was one of the things that made me want to do the film. It’s a little different from some of the stuff I have done, and I just hope it’s intriguing to people.” Originally called “Dorothy and Ted,” the film—narrated by Laura Dern—was originally supposed to be structured around Dr. Lewis’s investigation into infamous serial killer Ted Bundy (“The Campus Killer”), whom she interviewed in 1989. In the final cut, however, Bundy is only a small part of a much bigger story. “Bundy is this magnetic force field that draws you in,” explains Gibney, “and that’s why we left him for the end. By the time you get to him, you are able to see him not as someone who is so extreme, but as a part of a continuum. He is not so mysterious.”

Since his execution in 1989, Bundy has come to personify the very modern view of the serial killer as a charismatic monster, but Gibney takes a different view. “People are fascinated by Bundy,” he says, “because he seems to be the paragon of pure evil: the good-looking, charming man who had an idyllic childhood and was possessed by some demonic force we will never understand. Turns out that if you dig into the evidence, there are indicators of extreme mental disease and possibly a brain injury. His own ridiculous claims that he became a serial killer because of pornography—which so many people bought—I just find laughable. If pornography causes you to become a serial killer, we have a lot of them.”

Instead of spending yet more time on the well-documented Bundy, Gibney focuses instead on Dr. Lewis, whose careful and very human interview style he praises. “There is a certain style of interviewing that purposefully creates a distance between the interrogator and the subject. I don’t like that – I try to have a conversation.” As an example, he cites forensic psychiatrist Park Dietz, who has also testified in many high-profile cases. “It’s as if he was an interrogation machine. He couldn’t care less about that person as a human being,” Gibney alleges. “[But] Dorothy laughs, cries and she is very careful not to be shocked or contradict what they say. People she is interviewing are very defensive about being called crazy, and I find that interesting too—crazy people don’t like to be called crazy! What I also found interesting was her reliance on simple questions, rather than grand, broad philosophical ones.”

As well as dealing with the crimes of serial killers, Gibney’s film also addresses their punishment too—arguing that people need to re-examine their reasons for wanting a death penalty, the director points out that in the states where it’s legal, homicide rates actually tend to be higher.

“It’s serving as a kind of permission given by the society,” he notes. “We mention that in England, the law used to have this notion that being insane was punishment enough and yet we lost [that idea] along the way. Many people I talked to said: ‘What do I care what happens to a serial killer?’ The film doesn’t try to suggest that you shouldn’t be horrified by these acts. But what’s the point of making up fiction, imagining that such a person is evil as opposed to disturbed or mentally imbalanced, and maybe not in control? If the goal is for this to stop, then we should know what causes it.”