In the book, entitled "Zero Degrees of Empathy" in Britain, and "The Science of Evil" in the United States, where it comes out in July, [Simon] Baron-Cohen seeks to pick apart and define components of empathy -- including hormones, genes, environment, nurture, and early childhood experiences.
Citing decades of scientific research, he says there are at least 10 regions of the brain which make up what he calls the "empathy circuit." When people hurt others, either systematically or fleetingly, parts of that circuit are malfunctioning.
Baron-Cohen [who is also director of the Autism Research Center at Cambridge] also sets out an "empathy spectrum" ranging from zero to six degrees of empathy, and an "empathy quotient" test, whose score puts people on various points along that spectrum.
Drawing a classic bell curve on a graph, Baron-Cohen says that thankfully, the vast majority of humans are in the middle of the bell curve spectrum, with a few particularly attuned and highly empathetic people at the top end.
Psychopaths, narcissists, and people with borderline personality disorder sit at the bottom end of the scale -- these people have "zero degrees of empathy."
But rather than labeling them as evil, Baron-Cohen says they should be seen as sick, or "disabled," and we should seek to understand why they have such an empathy deficiency and help them replace it.
Baron-Cohen shies away from saying that psychopaths can be "cured" of extreme behavior, but he argues strongly against locking them up and saying there is nothing society can do.
The connection of psychopathy with a lack of empathy that distinguishes the acts of these people from others (sometimes it is characterized as a lack of conscience to avoid confusion of the ability of people with it to predict the behavior of others), is at the heart of the understanding of the condition as explained by multiple authors. Some reviewers of the field have noted, however, that the most frightening violent psychopaths seem to have both a lack of empathy and in addition some trait that inclines a conscienceless person towards violence rather than, for example, fraudulent manipulations of people.
Accounts I've seen elsewhere have also pointed to a very early childhood, if not congenital origin for the condition. Absent brain injury or something of that sort, psychopathy seems to be present at least by the time that a child is school aged.
Practical advice on dealing with these individuals can be found in this post.