PSYCHOPATHS
A psychopath is an individual who has an inherited neurological disorder which makes it impossible for them to grow a conscience. It doesn't matter whether the psychopath has good, loving parents and a middle-class socio-economic background (e.g. Jeffrey Dahmer) or suffered some abuse or psychopathic or sociopathic training; the psychopath only turns out one way no matter what you do - biologically human, psychologically monster. The point that seems to be most difficult to grasp for those just learning about this subject is that the psychopaths have all the faculties of human consciousness (including the Interpreter and Projection) except one - a conscience.
The conservative estimate is that they make up 1% of the population; in the U.S.A., with a population over 300 million people, that's at least 3 million psychopaths. Most of them are not recognized as such - except by their victims (and sometimes not even then).
The Psychopath derives pleasure directly from harming others, as an end in itself.
An SJ can be a psychopath, and will have all the qualities of an SJ and all the qualities of a psychopath. This is a real trap for the vast majority of SJ's, because they will focus on the way in which the psychopath is like them; their SJ-ness (faith in institutions and belief in the inherent goodness of everyone - "God don't make junk") will make them "refuse to see" the ways in which he or she is unlike them.
An SP can be a psychopath too, in which case the SJ's will much more easily recognize the fact; the SJ's will see that the psychopath SP, being an SP, is already unlike them and therefore the "psychopathy" will be easier to admit. For all others (SP's, NT's, and NF's) the psychopath SP is very dangerous; he or she can be very charming, and the lack of self-consciousness can seem like confidence to the un-burned.
SOCIOPATHS
The sociopath, as the name implies, was socialized to be that way. It is a man-made fact, and therefore could have been different, and (maybe) can be changed. There is a genetic component, in that all sociopaths are "sensation types" (specifically SP's), but they did not have to turn out that way, unlike the psychopaths.
The SP's - having an inherited preference for physical sensation instead of intuition, but not a preference for sticking to a schedule - have no inherent resistance to an Evil Culture (a Culture of Lying and of leaving people to fend for themselves). Not all SP's are sociopaths, but the phenomenon seems to be increasing with each generation.
In some ways, the Sociopath is like the Psychopath, only less so: lying, cheating, stealing, making promises to get what he/she wants then promptly forgetting what was promised in return.
The Sociopath uses others callously as a means to an end, but does not derive pleasure directly from harming others.
Anyone who feels free to do whatever they want - and is not a psychopath - is a sociopath. How many sociopaths are there in the U.S.A. right now? Could it be 10 million? Could it be more?
There are no SJ Sociopaths, but the SJ's very SJ-ness can make them support The Evil if the psychopaths and sociopaths can get control of the political and economic institutions. This is what has already happened.
EVOLUTION
There are no NT or NF psychopaths or sociopaths. The inherited defect of the psychopath precludes being Rational or deeply compassionate; whereas the inherited preference for intuition over being practical gives an inherent resistance to the "sociopathic training" of the Evil Culture.
SUGGESTED READING
The Mask of Sanity by Hervey Cleckley - the classic work on the subject - is out of print but available free online. Thank God for the internet! (And thank you, you krazy kassiopaeans.)
Available at libraries and bookstores everywhere:
Without Conscience
by Robert Hare.Snakes In Suits
by Paul Babiak and Robert Hare.The Sociopath Next Door
by Martha Stout.Cleckley, Hare, and Babiak all refer to both psychopaths and sociopaths as "Psychopaths"; Stout refers to both psychopaths and sociopaths as "Sociopaths". Every experience in this world is an opportunity to practice discernment.
Update 10/28/11!
The Heritability of Psychopathy: Implications and Considerations
