He says he can often see it in their eyes: There's an intensity in their stare, as if they're trying to pick up signals on how to respond. But the eyes are not an element of psychopathy, just a clue.

cores their pathology on the Hare Psychopathy Checklist, which measures traits such as the inability to feel empathy or remorse, pathological lying, or impulsivity.

The scores range from zero to 40

"Psychopathy has long been assumed to be a single personality disorder. However, there is increasing evidence that it is a confluence of several different personality traits,"

psychopathy appears to be a complex, multifaceted condition marked by blends of personality traits reflecting differing levels of disinhibition, boldness, and meanness.

Many psychologists also assume that psychopathy is inalterable -- once a psychopath, always a psychopath.