Sherlock’s dry response to policeman Philip Anderson’s remark about him being a psychopath — "I'm not a psychopath, I'm a high-functioning sociopath. Do your research" — is now one of the hallmarks of the show.

A new "Sherlock" special premiered January 1 on PBS.  And everyone’s been talking about it since!

We all wondered which would Sherlock really be, a sociopath or psychopath, and why.

Business Insider posed this question to James Fallon, a neuroscientist at the UC Irvine School of Medicine who specializes in studying psychopaths.

Fallon in turn asked his friend Michael Felong, a doctor who specializes in internal medicine in Temecula, California, who has an interest in Sherlock Holmes. Eager to know the answer?

Sherlock Holmes, as written by Doyle, is probably what he calls a "primary psychopath," not a sociopath, he said to Business Insider reporter.

The term "psychopath" doesn't appear in the latest version of the Diagnostic and Statistical Manual of Mental Disorders (DSM), the medical handbook used by psychiatrists. The closest entry isantisocial personality disorder, which is defined by "impairments in personality," such as egocentrism or lack of empathy, and "pathological personality traits," such as manipulativeness or impulsivity.

Psychopaths and sociopaths are sometimes considered the same thing, but there are some key differences between them, Fallon told Business Insider.

According to him, psychopaths can be divided into two categories: primary psychopaths and secondary psychopaths, or sociopaths.

A primary psychopath usually gets his or her defining characteristics as a result of a combination of genes, brain connections, and environment, said Fallon. This type of person doesn't typically respond to punishment, fear, stress, or disapproval, and often lacks empathy. Most primary psychopaths, Fallon added, mimic emotions and understand them cognitively, but do not feel them.

A secondary psychopath (sociopath) gets to be this way mostly as a result of his or her environment. Severe abuse at a young age can play a particularly strong role in the development of a sociopath, said Fallon. Unlike a primary psychopath, a secondary psychopath or sociopath can feel stress or guilt, said Fallon, and is generally capable of empathy. He or she may also be prone to anxiety, Fallon added.

Both primary and secondary psychopaths can further be divided into "distempered" and "charismatic" psychopaths, Fallon explained:

A distempered psychopath tends to fly into rages that can resemble epileptic fits. These people may also often have an extremely strong sex drive.

A charismatic psychopath is often a charming liar and fast talker who can manipulate others to part with anything — including their lives.

Now that we know the difference, let's take a look at Sherlock.

WHY SHERLOCK IS A SOCIOPATH?

Sherlock Holmes is a brilliant but antisocial detective. He doesn't seem to show emotion or care about other people's feelings — even those of his trusted sidekick Dr. Watson — and he's not driven by the fear of offending others. By all appearances, he is a primary psychopath.

What's more, he never loses his cool and seems to have very little interest in women (with the possible exception of his femme fatale Irene Adler), and yet he wins the admiration of Watson and his many fans, which probably makes him a charismatic psychopath.

That said, the Sherlock played by Benedict Cumberbatch in the MASTERPIECE/PBS series is perhaps a tiny bit more humane than Doyle's original character. He occasionally shows acts of kindness toward Watson, and despite his tough veneer, he betrays the tiniest glimpses that he cares about others.

But these changes were probably necessary to make him more likeable to audiences, Fallon said. After all, "real psychopaths are terrible characters.", reports the Business Insider.