What it's like to spend 20 years listening to psychopaths for science

WIRED: In the book you write that in two decades you’ve only come across a handful of people who scored 40 out of 40 on the psychopathy checklist (30 is commonly used as the psychopath cutoff. Regular folks tend to score around 4 or 5). What is it like to talk to a perfect psychopath?

Kiehl: They are so fundamentally different. You leave the room knowing that you’ve just met someone who is extremely different, even different from other psychopaths. They are absolutely and completely free from conscience. They have this unbelievably flat affect that’s really palpable when you look in their eyes.