John Velez is an assistant professor of journalism and electronic media, and he set out to study the psychological effects that cooperative video games can have on players. Velez observed participants who played through violent games like Halo: Reach and Time Splitters, as well as non-violent games like NBA Street Homecourt, and he found that those who had a helpful partner tended to be more pro-social both during the game and afterward.
"What we found was cooperative play seems to have the biggest effect in terms of decreasing aggression toward other people," Velez said. "We found that playing with a helpful partner increases the expectation of others to reciprocate that pro-social behavior and generally be helpful. That applies to not only the teammate, but to others as well."