Empirical Evidence of Something Gamers Have Known All Along

As the old joke goes: violent content doesn't make gamers aggressve; lag does.

http://www.bbc.com/news/technology-26921743

The research sought to establish whether it was violence in games which made players feel more aggressive, or a combination of other factors.
Six separate studies were carried out.
One of them involved modifying Half-Life 2 - a critically-acclaimed, but graphic, shooting title.
The researchers created a modified version in which rather than violently removing enemies, the player would instead "tag" foes who would then evaporate.
This version was tested alongside the normal, violent version.
However, only some of the gamers were given a tutorial before playing the game so they could familiarise themselves with the controls and game mechanics.
The researchers found that it was the players who had not had the tutorial who felt less competent and more aggressive, rather than people who had played the more violent version of the game.