David Geary did a study with 42 male students at the University of Missouri. The students, who did not know each other, were divided into 14 groups of three. Each segment was then tasked with practicing Epic Games’ top-rated frenetic first-person shooter Unreal Tournament 2004 for six hours over a weeklong period, in order to establish familiarity with teammates.
Teams were then pitted against one another in 30-minute Onslaught matches, a capture-the-flag variant. When matches were completed, Geary and his colleagues found that testosterone levels dramatically rose in members of the winning team, especially those who contributed most significantly. However, in Death Match matches, where each team was pitted against itself, researchers found that the highest-ranked player saw a significant drop-off in testosterone levels.
"In a serious out-group competition, you can kill all your rivals and you’re better for it," Geary said of the study’s findings. "You can’t alienate your in-group partners, because you need them." The study also indicated that multiplayer games trigger the same biological instincts as actual warfare, where heightened testosterone-induced aggression can be beneficial.