Psychopaths = mentally handicapped

One researcher has found that psychopaths may be more aggressive than the average population because they do not assign meaning to the fearful faces of their victims:

In her groundbreaking work funded by the National Institute of Mental Health, [Abigail] Marsh and her colleagues have been exploring how “callous and unemotional” individuals tend to show a very specific cognitive deficit: namely, they are especially poor at recognizing, processing and responding normally to the facial expression of fear on other people’s faces (a “normal” response being ceasing an assault on the frightened person or offering aid). Curiously, their trouble in this area is not due to a problem with facial expressions in general—they do perfectly well deciphering the look of disgust, anger, happiness and so on on other people’s faces. In contrast, autistics have trouble with pretty much all facial expressions of emotion, suggesting that, for them, this generalized difficulty is meaningfully linked to their broad social disfunction. Rather, it’s only the look of fear that puzzles diagnosably antisocial people (and to a somewhat lesser extent, sadness). Thus, in a converted boathouse on Squam Lake in early July, Marsh discussed several key studies, all indicating a fear-specific facial processing deficiency in children and adults with persistent antisocial behavioral tendencies. That is to say, “behavior that violates the rights and welfare of others or breaks important normative rules.”
. . .
Marsh relayed a chilling anecdote about a colleague of hers, University College London psychologist Essi Viding, who was going through a task with a psychopathic murderer in which a series of faces with different emotional expressions were laid out before the woman. When the murderer saw the picture of the fearful face, she scratched her head and said: “I don’t know what that expression is called, but I know it’s what people look like right before I stab them.”

Interestingly, another study found that the people who can recognize fearful faces recognize them faster than any other "emotion" face. I guess that makes psychopaths literally "slow."