Through a study of American psychiatrist Dr. Dorothy Otnow Lewis' life and work, Academy Award-winning documentarian Alex Gibney explores why people become serial killers.

Serial killers are, by most peoples' standards, the embodiment of evil.

But we are often far better at labeling people than we are at understanding them.

American psychiatrist Dr. Dorothy Otnow Lewis has studied 22 serial killers over the course of her career and made it her life's work to develop a better understanding of what drives such people to act in violent and murderous ways.

Lewis' research suggests that serial killers commonly suffer from dissociative identity disorder (or D.I.D.), previously known as multiple personality disorder.

Lewis has provided expert evidence in numerous high-profile murder cases, arguing that those who suffer from D.I.D. should not be executed for their crimes.

Directed by Oscar-winner Alex Gibney and narrated by actress Laura Dern, Crazy, Not Insane explores the controversial scientific and moral issues surrounding society's understanding and treatment of serial killers.