"I'm not a psychopath, Anderson, I'm a
high-functioning sociopath. Do your research."
In this scene from S1E1, A Study in Pink, Sherlock claims to be a sociopath. The impossibility of Sherlock being either psychopath or sociopath is addressed in Psyching Sherlock: the Impossible Sociopath. But he does tell Anderson to "do research" and that research can hold, for the viewer, the key to the defeat of Moriarty on the roof of Saint Batholomew's Hospital in S2E3.
Moriarty as written and as portrayed by Andrew Scott is a psycopath, and he also seems to be a bit crazy. These two things don't necessarily go together, and so it reasonable that it took Sherlock a while to figure this out. ("You're just getting that?" Moriarty to Sherlock on the roof of St. Bart's when Sherlock declares: "You're insane.") As the quote below tells us, psychopathy and psychosis rarely overlap:
People often confuse the idea of psychosis with psychopathy or sociopathy, or think that all psychopaths are psychotic. These disorders are actually very different, and rarely overlap (link)Moriarty is unique in Sherlockworld, described by Mycroft as "the most dangerous criminal mind the world has ever seen," in S2E3, The Reichenbach Fall. A high-functioning psychopath of extreme intelligence and also insane.
Psychopath vs Sociopath
While many authorities recognize no difference between a psychopath or a sociopath, others make distinctions based on the cause of the behavior:
Some separate psychopathy and sociopathy based on their proposed causes. For instance, some people say that a person is a psychopath if he or she developed psychopathic characteristics primarily because of a genetic predisposition, and a sociopath if he or she developed the characteristics primarily in response to environmental factors, like abuse. (link, scroll to Proposed Causes)The difference here would be that a genetically-linked psychopath would have a physical difference in neurology and be impossible to treat successfully, whereas a sociopath, if the root of the disorder were determined, could respond to traditional therapies. That is: one can change while the other cannot. From the British Journal of Psychology:
THE NEURAL BASIS OF PSYCHOPATHY
Sherlock, during the course of the two Series, changes. He gets better. Possibly because he was never a sociopath at all, as argued in the "Psyching Sherlock" post, or because his condition is a function of environmental factors that are being overcome through his relationship with John Watson.
|Moriarty: "You're not ordinary. You're me."|
Information is power. Moriarty didn't know he and Sherlock were different. He believes Sherlock is just as desirous of death, just as sick of life ("our problem, staying alive") and looking for an excuse to end it. But Sherlock has always known he is different: "I'm not a psychopath..." while still understanding the mind of one who is. He uses Moriarty's tactics against him: gives him an excuse to die and lets him think he won.