A researcher down in Texas has found that, when people are assigned to play characters wearing the brooding black robes of an emo Jedi, or the coneheaded getup of a KKK grand kobold or whatever, they exhibit antisocial behaviors.
The study, entitled "The Priming Effects of Avatars in Virtual Settings," more or less says that people don't have to self-select antisocial depictions of themselves to engage in antisocial behaviors. Just being told to play on the Dark Side in Jedi Knight II, or as a Klanketeer in another setting, encourages them to be a-holes. So you can imagine what that says about a person who creates the skeleton-faced, top-hatted all-black avatar with a tag like xXx_ScAbIEsDeM0n_XxX.
Before you get huffy, the study's author isn't singling out video games or avatars; they just comprise one environment that can also "prime" people to be louts and douchebags. Let me submit another: The 101 from the 87 to Mountain View at 8:25 a.m. on a weekday.
But yeah, as "no shit" findings go, it does sound rather obvious. You can read up on the rest of the study if you like.