Hollywood tropes of "sociopaths" have built them up as fiendish, moustache-twirling evil Magnificent Bastard Villain Sue masterminds with an Improbably High I.Q., however, this doesn't quite reflect reality. In particular, the famous "Hannibal Lecter" archetype has heavily influenced subsequent characters, but Lecter is more of a comic book villain - real sociopaths don't act like that. The reality is in some ways much, much worse. While to the casual observer going about their everyday business a sociopath "seems normal" and often possess a superficial charm that can allow them to pass as normal people, if you spend any length of time with them or know to be looking for sociopathic indicators, their "Mask of Sanity" won't hold up under high scrutiny. Essentially, it is a very insulting Clark Kent Outfit that sociopaths mentally put on when they try to blend in with society: they perceive normal people as a bunch of gullible, sentimental fools who slather each other with token pleasantries like "Hi, how are you? Looking good today, have you lost weight?" and then honestly put their "trust" in other people, based on these shallow pleasantries. A feature of this is "semantic aphasia" — sociopaths don't associate the same negative meanings to words as normal people. Thus you might hear a sociopath, honestly trying to keep his cover and blend in with normal society, suddenly crack a joke that they think is hilarious about rape, murder, torture, etc, that a normal person would find offensive and inappropriate. It is common for normal people to think that a sociopath just doesn't understand the full moral implications of what they're doing, that if they thought they were "bad" they wouldn't do them. However that is one of the reasons why they do it and why they enjoy it. They are malignantly antisocial, and the worse it is, the more they enjoy it. On a broad, theoretical level, they sort of understand that they're not supposed to admit to murder, rape, torture, dismemberment, arson, cannibalism, etc. but the exact line between "morally acceptable" or not is very blurred. Essentially they have learned to mimic the rules of the society that they live in, but little mistakes will give them away. In interviews with real-life serial killers like Ted Bundy, John Wayne Gacy, etc. they seem to have surprisingly normal speech patterns, they're not ranting or raving, but if you pay close attention over a long period of time, every now and then they make little slips where they say bizarre or disturbing things — because they're just pretending to be normal, its all an act. Sociopaths do not necessarily have some sort of Freudian Excuse for their Lack of Empathy (although if they get caught out with a lie they will invent one). However, many works will have such characters grow up with horrific abuse, conditioning them to believe that they live in a nihilistic dog-eat-dog Crapsack World where empathy is useless. One theory of sociopathy is that usually those who have it did not merely have Abusive Parents; rather, they had Abusive Parents who periodically would make some half-assed and short-lived attempt at discipline or generosity, or pretend that everything was "normal", for example to fool outsiders. It's the chaotic and random hypocrisy of the situation that really set them off, teaching them that punishment is arbitrary, kindness is a mask, and other such Family Unfriendly Aesops that they end up internalising, along with a shortage or total lack of more positive role models, which eventually leads them to adopt a mentality not unlike "Then Let Me Be Evil". This is partly because many people can't see why somebody would be so 'cold' and 'ruthless' without a good reason. But sociopathy generally does not have any better reason to be there than any other sickness (although negative experiences with other human beings will change the way it manifests itself). Others have theorized that they often have abusive upbringings because their parents have the same mental illness as their children. Sociopaths will often try to play the sympathy-ploy that they are the way they are due to abuse or trauma they suffered...but the reality is that even small children who grow up experiencing the War Is Hell environment in all its horror, subjected to truly nightmarish experiences, don't turn into sociopaths (and in fact may turn even more empathic to people suffering the same hell). Sociopathy is a personality disorder. While an abusive childhood will often make their behaviour worse and help it manifest, it is not the cause. Then again, some sociopaths reportedly have completely normal upbringings, with seemingly no provocation for their behavior. In real life, medical tests conducted on sociopaths show that they're often unable to recognize pain or suffering in others - they're not necessarily evil, they just lack the ability to "connect" to other people's feelings even if they understand them on a rational level. In fact, Cleckley notes that often the greatest harm that they bring to others is the worry and sleepless nights that they create in family members from their frequent unexplained absences, irresponsible and reckless behavior. Sociopaths are far more likely to be common, callous petty criminals, bullies and abusers than they are to be serial killers. That said, an "evil" person would probably score highly on a sociopathic checklist. Brain scans have determined that there is indeed something structurally wrong with the brains of sociopaths. They have structural and functional abnormalities in their amygdala, the deep part of the brain responsible for generating emotions, and certain regions of their prefrontal cortex, which is involved in learning and the processing of higher emotions such as Empathy. They can't comprehend empathy. Heavy use of narcotics is associated with the development of sociopathy, because the drugs cause brain cell apoptosis (cellular death), causing brain lesions. Brain damage to their prefrontal cortex and some neurodegenerative diseases that affect the prefrontal cortex can cause a condition known as acquired sociopathy. Genetics are also believed to have a large role to play in its development, and sociopathic traits in children who have otherwise healthy childhoods, known as callous and unemotional traits, have been detected from an early age. They can use the rational part of their intellect to play-act or mime their way through imitating complex emotions, but they don't truly feel them, and thus can't really understand them. Besides pretentiousness and an utter lack of empathy, the other major defining feature of sociopaths is extreme impulsivity. Impulse Control Disorders have been shown to be caused by frontal lobe damage. They frequently complain of being "bored" and thus doing it "For the Evulz" — psychologists suspect this is due to a lack of serotonin which causes the ever-present boredom and the seeking out of "exciting" activities, or because their damaged amygdala also structurally prevents them from restraining their impulses. Given that socialized traits like impulse control and ethical norms are initially learned by children through stimulated emotional responses, such as learning to associate breaking rules with fear or anxiety, and normally always evoke those same emotional responses at least to some degree throughout life, it follows that people who can't experience emotions in a normal way likewise can't readily learn through processes that rely on emotional responses. The behavior of a sociopath also seems to make very little sense to those who observe them. An example would be a patient at a psychiatric facility who spends weeks trying to get grounds privileges. Once they have these privileges, they do something so obvious that a sensible person would expect to get caught, and which results in little gain, like escaping only to get into a fight at a nearby bar. It appears that they have just squandered something that they worked weeks for, for only a slight bit of amusement. This results because their cognitive abilities (i.e. ability to sort through information immediately coming from the surrounding environment) is limited by an inability to take responsibility and long term goals into consideration, making them unable to learn from punishment. Whatever the cause, this leads to one of the major disconnects between the Hollywood "evil supergenius" sociopath and the real-life kind: true sociopaths tend to be very bad at long-term planning. They cannot take the past or future into account; only the immediate present, and, thus, they largely lack the ability to delay gratification in pursuit of larger ends. They follow poorly planned out whims, purely focused on the immediate gratification and with zero thought of the consequences...even to themselves. Even when lying or bragging would only get them into even more trouble. Paradoxically, this can make sociopaths, once identified as such, easier to predict than average people. Their behavior will usually be directed toward the shortest and easiest perceived routes to their own immediate personal needs or wants, and will be unaffected by many of the varied, complex, and often conflicting constraints, influences, and internal and external judgments that affect most people's behavior. Conversely, their lack of empathy will also typically leave them not only apathetic to how other people are likely to react to and judge their behavior, but very often also poorly able themselves to accurately predict and understand other people's behavior and reactions. This might be surprising considering how well sociopaths are often able to charm normal people in long-cons, blending into society...but a charming used-car salesman doesn't really need that much long-term planning. This is because a sociopath's skills as a Manipulative Bastard is based largely on improvisation: they custom-tailor their lies spur-of-the-moment, without much thought as to how all of their lies will fit together into a cohesive whole. Not that they care, they'll just think up a new lie to patch over the inconsistency, but over time they add up (to put it in troper-speak, this is the same kind of repetitive lying that drives a "Fawlty Towers" Plot, the biggest difference being that a sociopath will continue building even as the lie is in the process of collapsing). For example, when Ted Bundy was asked if he ever did drugs, he launched into a long and rambling diatribe, first adamantly insisting that he never did, then that he might have tried marijuana, then admitting that he took marijuana and might have tried crack, but yeah, he still feels he "doesn't do drugs". Each individual sentence was "acted" wholeheartedly in their respective moments, because he was a shameless liar, but if you paid attention to the logic of his words, they were incredibly incoherent. John Wayne Gacy adamantly insisted in court that he was innocent, but when victims on the stand described how he tortured and nearly killed them, he started laughing in open court, sort of how a normal person might laugh at a man getting hit in the groin by a football — he couldn't comprehend that this was inappropriate to normal people, nor did he realize how absurd it made his defense look. Thus a sociopath can maintain the mime-act of pretending to be normal for years, but this isn't really a "long-term plan" so much as they're so impulsive and willing to lie, that they just make up new lies every day. The reason for this charm is fairly simple: Having zero remorse and the ability to discard anything if it does not suit them, they have absolutely no fear of lying and/or honestly believing that Two Plus Two Makes Five, and this makes them very good actors. A major component of sociopathic lying is not appearing nervous, and being truly "shameless", sociopaths will tell you exactly what you want to hear, and will be unconcerned if they do get caught in lies. Pathological lying is caused by damage to the frontal lobes to the brain, so their compulsive lying is also a symptom of their illness. Additionally, sociopaths' emotional abilities are so stunted that their cognition and language are affected in characteristic ways. They're often unable to effectively grasp the emotional connotations of words, and so often use words with connotations that are either too sparse or else too florid to be appropriate in a given context, misuse or misunderstand words, or create idiosyncratic neologisms. Their speech also regularly invokes superficial, hollow expressions of trite sentiment; and they're also often unable to maintain a coherent narrative in their communications when pressed for detail, instead digressing into unresponsive tangents (or, more to the point, tangents responsive only to what they want to talk about). Their manipulative glibness, the audacity of their lies, use of Dirty Social Tricks, and effective delivery often prevents their victims from catching on to how creepy and manipulative this kind of communication actually is. It occasionally happens that even experienced psychologists studying probable sociopaths in custody get taken in by their manipulation in interviews, only to later be shocked and embarrassed by how obvious the attempted manipulation was after reviewing the recordings of the interviews as well as clinical and criminal histories. Also, beyond being merely a functional necessity and outgrowth of their personality, sociopaths' compulsive lying is also driven by personal enjoyment at the thought of being able to manipulate people, called "duping delight" by at least one clinical psychologist. Their egocentric worldview is fed by the perception (whether accurate or not) that they are able to manipulate others easily. Their compulsive dishonesty and grandiosity also very often leads them to pretend to expertise and knowledge in particular fields, such as psychology, philosophy, law, medicine, finance, business, science, or the arts, in ways that sound convincing enough to give a good "two-minute opening speech" to lay persons (which often aids them in running cons or other fraudulent schemes), but which readily collapse in the face of actual expertise. It's common for sociopaths who are in trouble to try to "match wits" with clinicians, law enforcement, correctional authorities, lawyers, judges, and other trained specialists, and to try to represent themselves in court or other legal or administrative proceedings, often with shambolic results. (Ted Bundy, for example, insisted on retaining control of his own legal defense during his final murder trial despite the involvement of five court-appointed attorneys, and refused a plea bargain that would have kept him off death row and possibly allowed him a later appeal. He was convicted, sentenced to death, and died in the electric chair.) TL;DR: We think that, for sociopaths, everything goes "Just As Planned" when in fact that's what they want you to think. An exception to this cognitive weakness can exist in the more intelligent types of sociopaths who do know how to Cut Lex Luthor a Check and use their skills to rise in the social hierarchy. The combined result is possibly one of the most frightening aspects of the most hardcore of sociopaths: Because they're so apathetic, impulsive and unfettered, they have an inability to learn from past mistakes and it becomes impossible to punish them, and they never learn to fear punishment of any kind, even when it would mean their own destruction. The Sociopath never seems to see that there's a problem with the way he treats others and as such doesn't admit he has a problem, even if you humiliate him and kick him out - so it's tough to teach him a lesson. The only reason he'll stay in somebody's life is because that person is gullible and there's no need to reinvent the wheel. Thus criminal recidivism (re-offending) is highly correlated with sociopathy (On the plus side, though, this can cut both ways and make the sociopath prone to failing because they Didn't Think This Through; they might attempt to commit the same type of crime on three different occasions only to be foiled in the same exact way every time since they forget about what doomed them the last time). It has been shown that punishment and behavior modification techniques do not improve the behavior of a sociopath; in fact, such techniques only make the sociopath learn how to be more adept at manipulation and concealing their behaviour. They are almost impossible to cure as they often see their therapist as just another person to con, and may well persuade them that they have been cured when they still don't see a problem with their past behaviour. If you do try to lecture and psychoanalyze an intelligent sociopath, he will lecture and psychoanalyze you back. And chances are he'll win (or perceive himself as winning, which for him is the same thing). A common tactic that sociopaths fall back on, after all other possible lies have been exhausted (and that does take a while, for them) is to flat out insist that their accuser is simply delusional or insane, i.e. deny that their hands are visibly covered in blood. This isn't because they're being petulant or stubborn; they genuinely can't mentally process a scenario in which they are at fault (though in some cases they do know, and this is a calculated tactic to simply bully their accuser into submission). Moreover, given the defective ability to understand other people implied in their lack of empathy, what "understanding" they do have is usually thoroughly shot through with Psychological Projection: they will readily accuse other people of the exact motives and behaviors they themselves are displaying. They can decide that Two Plus Two Makes Five, and all of a sudden every calculator in the world is wrong. If a Spanner in the Works ruins their plan, they chalk it up to fate, bad luck, or other outside interference, even if the spanner was inherent to the plan itself. To them, whatever they think of something is how it objectively is in the realm of reality; the fact that they can't beat a trained boxer in a fistfight is merely because they have the most ridiculous run of bad luck with lightning having (metaphorically) struck them 15 times. Those silly laws of physics that allegedly state they're incapable of walking on thin air? Please, that's just a cover-up by The Man that all those gullible fools (a.k.a. every single other person alive) believe. And they genuinely, wholeheartedly believe it. This makes them different from other personality disorders associated with shamelessness, impulsivity, lack of empathy and an It's All About Me mentality, such as the Extreme Narcissist: when in doubt, narcissists want others to take care of their needs and wants, while sociopaths will just take what they need or want. Narcissists cannot really dispose of their emotional, moral and conscientious weaknesses, such as love or guilt, and when cornered, are far easier to humiliate. On the whole, however, they are completely impulsive and will keep doing bad things...because possessing no empathy, there is no greater rush for them than to cheat and harm other people. Being utterly impulsive and lacking any emotional and rational restraint, they'll keep doing harmful, self-destructive things. Ted Bundy and John Wayne Gacy killed dozens of people, not as spur-of-the-moment crime of passion, but consistently over many years...because they honestly enjoyed doing it, thought of their victims as simply objects for their amusement, and would never stop. For them, a lover is just another potential victim. Any kids are simply a burden - or at best a tool. They will often run away if they are faced with any responsibility for their family. However, sociopaths are capable of faking affection for a time, which will seem authentic to many. Love is merely a game for them, and they will essentially see their lover as simply a prize to be won, or a possession to be owned. While The Sociopath is often associated with other Above Good and Evil personalities, many examples with morals also exist, but they are more likely to suffer from Moral Myopia, or subscribe to a self-centered moral code. These kinds of behaviors cause The Sociopath to be nigh-universally shown in fiction as remorseless, unsympathetic, and unfettered, and he (and it is almost always a he, as female sociopaths are extremely rarely diagnosed in real life) is almost never redeemable, with no remorse for their actions or the pain they cause others and take a sadistic pleasure in it; we very, very rarely get a sympathetic or redeemable sociopath. If a sociopath is portrayed as attractive or "sympathetic", it is less from actual sympathy and more from Magnificent Bastardry and other examples of Evil is Cool. This is unfortunately frequently Truth in Television, as they often inflict repeated harm on both those close to them and strangers. A diagnosis of Antisocial Personality Disorder or Dissocial Personality Disorder, as sociopathy is known in psychiatry, is only applied to adults; if the individual in question is under the age of 18, they would receive a diagnosis of Conduct Disorder, possibly with a specifier for Callous and Unemotional Traits. Sociopathy as a diagnosis is mainly a semantic and legal issue, though it is also relevant in research. Many professionals have tried to cure them or at least to get them to withhold their impulses with various kinds of methods and techniques. While some of them can be conditioned to keep their behaviour in check for a while, it is only out of fear of punishment and getting caught. They only act morally as long as they're rats in a Skinner Box. When the Skinner Box's rewards disappear, the self-restraint behaviour goes extinct. Their recidivism rate (how often they repeat crimes) is still universally high. It does not change what they are. Sociopathy is ultimately a personality disorder, a class of mental disorders that is ingrained in their basic character, and has up till now proven extremely difficult to treat. A treatment that would cure them would likely have to be something that does something biological that would get their brains to work in a healthy way. There is a great deal of misunderstanding these days as to the overlap and meanings of the words "sociopath" and "psychopath". The two terms are often used interchangeably, but they are not equal. "Sociopath" is no longer considered a useful word, because the term was originally also applied to people with autism ("sociopath" in fact means "social illness", and it could arguably be applied to both groups on that basis) or Asperger Syndrome (which was originally called "autistic psychopathy"), and wasn't very useful as a category. "Psychopath" (meaning "mind illness") has superseded and replaced it in medical fields. Additionally, from the Useful notes page on Lack of Empathy: Publicly, sociopaths, psychopaths, and people with Antisocial personality disorder (ASPD) are thought of as the same thing. Officially, there is a distinction, albeit a very blurry and not entirely consistent one.
Psychopathy is not a diagnosis that can be found in the DSM, which instead recognizes ASPD. However, it is an important concept in forensic psychology as a means of risk assessment, and is used to inform legal decisions regarding criminal offenders' sentencing or parole hearings. Much like "insanity", "psychopathy" has more legal connotations than clinical ones. It's also this guy specifically that's described in The Sociopath trope page.
ASPD is the DSM's answer to psychopathy and was established out of concern that the diagnostic criteria of the psychopathic personality was too subjective. Because personality characteristics cannot be assessed objectively, ASPD diagnostic criteria disregards most of them and instead is based predominantly on the presence of a consistent, long-lasting pattern of antisocial behavior. As chronic antisocial behavior is not exclusive to a single disposition, this means that the diagnosis of ASPD covers a significantly larger population than that of a psychopathic personality — most psychopaths fit the criteria for ASPD, but most people with ASPD would not be considered psychopaths according to the PLC-R.
The difference between psychopathy and sociopathy is fuzzier. The term was introduced in 1909, reflecting the new suggestion that psychopathy was largely a product of social factors. It eventually came to be used to describe "secondary" psychopaths — people who shared many characteristics with "primary" psychopaths, but were a product of severe social maladjustment and were capable of experiencing emotions of depression or fear. However, there is no official diagnosis of sociopathy/secondary psychopathy, and its distinction from psychopathy is more a matter of preference than fact.
Please note that there seems to be very little consensus on what the difference between a sociopath and a psychopath actually IS. Don't be surprised if you run into a definition that contradicts what has been said here. (And then something else that contradicts THAT definition, etc.)