Psychopaths Among Us, an article I actually meant to post when I read it on Saturday.

For his first paper, now a classic, Hare had his subjects watch a countdown timer. When it reached zero, they got a "harmless but painful" electric shock while an electrode taped to their fingers measured perspiration. Normal people would start sweating as the countdown proceeded, nervously anticipating the shock. Psychopaths didn't sweat. They didn't fear punishment -- which, presumably, also holds true outside the laboratory. In Without Conscience, he quotes a psychopathic rapist explaining why he finds it hard to empathize with his victims: "They are frightened, right? But, you see, I don't really understand it. I've been frightened myself, and it wasn't unpleasant."