It's become common for movies to refer to a killer as a 'psychopath' or a 'sociopath' - so much so that the two words are used interchangeably. However, they are NOT the same thing. Read on to understand more about the scientific basis for all that movie-world pop psychology...
Antisocial Personality Disorder (ASPD)
Both psychopathy and sociopathy are ASPDs, and are surprisingly common: 0.5 - 5 % of people in the USA are psychopaths or sociopaths, putting the total at between 1.5 and 15 million people!
There are similarities between the two - like any human condition, they are a product of both genetic and environmental factors - but interestingly, psychopaths are more a result of dyed-in-the-wool genetic predispositions ('nature'), whereas sociopaths are created almost entirely by social and developmental conditioning ('nurture').
How to Spot a Psychopath
Screen example: Hannibal Lecter
Social Traits: Deceptive, manipulative, skilled at controlling people and situations (despite what movies tell us, this is NOT linked to intelligence - narcissism and aptitude at exploiting others can be found in non-brainy people, too)
Ethics: No moral sense - 'right' and 'wrong' abstract, impenetrable concepts
Emotions: No emotions, no empathy, cannot have emotional relationships
Crimes: Premeditated, cold, controlled
Motives: Asserting dominance, sadism, personal gain
How to Spot a Sociopath
Screen example: Dexter Morgan
Social Traits: Irritable, withdrawn, antisocial
Ethics: Understand right and wrong - they just don't care
Emotions: Feel emotions, but very little empathy - i.e. they have their own emotions, but don't truly realize or care that other people's are comparable and just as valid, capable of human relationships
Crimes: Unplanned, impulsive, spontaneous, uncontrolled
Motives: Response to emotional impulses such as rage or revenge