"Someone with an undersize or underactive amygdala may not be able to feel empathy or refrain from violence."

"For example, many psychopathic adults and callous children do not recognize fear or distress in other people’s faces. Essi Viding, a professor of developmental psychopathology at University College London recalls showing one psychopathic prisoner a series of faces with different expressions. When the prisoner came to a fearful face, he said, 'I don’t know what you call this emotion, but it’s what people look like just before you stab them.'"

From an excellent article in The Atlantic called "When Your Child Is a Psychopath/The condition has long been considered untreatable. Experts can spot it in a child as young as 3 or 4. But a new clinical approach offers hope."

Also: A 6-year old who practiced killing her stuffed animals, choked her 2 year old sister, and, caught before the sister died, clearly stated that she knew what she was doing would kill and, further, “I want to kill all of you.”