Research conducted at OSU has found that players who engage in cooperative play in games show increased cooperative behaviour later - as opposed to aggressive behaviour:

The first study had students play the video game Halo 2 for 20 minutes in separate rooms, with each given four contrasting play conditions. In the cooperative condition, subjects played the game as teammates against computer aliens. In the direct competition condition, they played against each other. In the indirect competition, they played the game solo. In the last condition, they did not play the game until much later, after they filled out a survey for measures of cooperation.
The second study had students play Unreal Tournament 3 who were only placed in the cooperative or direct competition conditions. The second subject wore an Ohio State or Michigan T-shirt, demonstrating an in-group or an out-group manipulation. This, researchers say, had no effect on the results.
After both studies concluded, students took part in a cooperation task where each were given four dimes to start off. They were told that they could either keep all of the dimes or give them to another player. Each dime given to another doubled in value.
"The idea is that you can be selfish and keep your dimes or you can give them away, and if each person gives their dimes away they get more money so that’s the measure of cooperation," Ewoldsen said.
Both studies found that when they played cooperatively, people cooperated in the later task.
At the end of it all Ewoldsen says the best way to describe the results is that, "it’s not the content of the game that matters, it’s how you play the game that matters."
Ewoldsen said that he wanted to show that the relationship between video game aggression and actual aggression is more complex than it is made out to be by other research.
"Video games have become such an important part of adolescent socialization," Ewoldsen said. "Certainly the research has shown that playing violent games can have very bad effects, but it’s a much more complex relationship than that."
Source: The Lantern

http://gamepolitics.com/2012/09/25/research-coop-players-are-less-aggressive