The Greater Internet Fuckwad Theory ("GIFT"), first given a name by Penny Arcade, is a theory that tries to explain why many people seem to become anti-social Jerkasses online in spite of any in-person behavior.
This phenomenon has been studied in an academic setting; by all accounts, the comic's satirical analysis is spot-on: Normal people become more aggressive when they think their behavior carries no real-world social consequences. When people do not have to worry about getting in trouble with their loved ones, school, or place of employment, or getting into a direct physical altercation with anyone, they feel they have nothing to lose by being shameless, insufferable jerks behind the veil of anonymity that is The Internet.
This leads to a (depressingly) large number of people thinking that cyberbullying is funny—and that the Internet is the perfect place to spew all the bigoted, hypocritical, provocative, or otherwise hateful bile they would never say in-person. The GIFT also contributes to the pervasiveness of cyberbullying amongst young children and teenagers, which has led to suicides. The academic name of the phenomenon is the Online Disinhibition Effect. It should be noted that the Online Disinhibition Effect cuts both ways: While some people become jerks, other people discover that the anonymity and lack of consequence on The Internet allows them to be more honest and talk about issues which, under normal circumstances, they would be unable to address. These issues can be as simple as liking a movie, show, or musician normally seen as outside their demographic, as serious as mental health issues, or somewhere in the middle like an embarrassing sexual fetish. In any case, the exact same anonymity that turns some people into Jerkasses allows them to speak things that they might want to talk about but be unable to do normally for fear of social stigma.
The idea of anonymity's irresistible corruption due to lack of consequences and escape from restraints caused by being watched is extremely pervasive. Think of the people who leave unholy messes and/or graffiti in public restrooms. Nobody sees them do it, and they almost certainly would not make such a mess in their own bathroom where they would have to clean it up (hopefully), or when they're a guest in someone's home. The basic idea was first proposed by Plato in The Republic, where he recounts the myth of the Ring of Gyges, one of the original Invisible Jerkass stories. At least one psychology paper has confirmed that Internet anonymity is attractive to narcissistic, psychopathic, and sadistic people who take pleasure in making others suffer.
The rise of newer social media networks such as Facebook and comment feeds on certain news sites, which require people to attach their real name and maybe even other details about themselves to any posts they make, has caused a re-evaluation of the theory. Plenty of people seem quite willing to act just as obnoxious, rude, bigoted, and abusive while posting under their real identity as they would if they were posting under a pseudonym. A lack of anonymity might dissuade some people from being jerks, but it does not appear to effectively push people into good behavior as was originally thought.
Fun fact: The German, Norwegian, Swedish, and Danish word gift means "poison", which seems...oddly appropriate. The page in Spanish is T.O.N.T.O..
This is a Subtrope to Serious Business and Internet Backdraft. Vocal Minority is from where most of this comes. See also Internet Tough Guy, Troll, Garbage Post Kid, A Darker Me, Became Their Own Antithesis, The Gadfly, and Griefer for specific forms of this, or any comment section. Compare to What You Are in the Dark.