TheClockworkSoul writes "According to NewScientist, victorious gamers enjoy a surge of testosterone — but only if their vanquished foe is a stranger. Interestingly, when male gamers beat friends in a shoot-em-up video game, their levels of the hormone plummeted. This suggests that multiplayer video games tap into the same mechanisms as warfare, where testosterone's effect on aggression is advantageous. Against a group of strangers — be it an opposing football team or an opposing army – there is little reason to hold back, so testosterone's effects on aggression offer an advantage. 'In a serious out-group competition you can kill all your rivals and you're better for it,' says David Geary, an evolutionary psychologist at the University of Missouri in Columbia, who led the study. However, when competing against friends or relatives to establish social hierarchy, annihilation doesn't make sense. 'You can't alienate your in-group partners, because you need them,' he says."