‘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.”

In another Hare study, groups of letters were flashed to volunteers. Some of them were nonsense, some formed real words. The subject’s job was to press a button whenever he recognized a real word, while Hare recorded response time and brain activity. Non-psychopaths respond faster and display more brain activity when processing emotionally loaded words such as “rape” or “cancer” than when they see neutral words such as “tree.” With psychopaths, Hare found no difference. To them, “rape” and “tree” have the same emotional impact — none.’