When you beat someone in a competition, you experience a surge of testosterone. However, a study suggests this is only true if you've beaten people you don't know. Researchers at the University of Missouri in Columbia claim the testosterone surge is greatly reduced when friends beat each other.

This article at New Science has a surprisingly detailed breakdown of how the study was conducted using Unreal Tournament 2004's Onslaught mode. Games were played among strangers who were divided into teams that practiced with each other and presumably became friendly. The players were paid, but to raise the stakes, the winners were paid more.

[Researchers] found that testosterone levels of the winning team members spiked immediately after the tournament, particularly among players who had contributed most to their team's victory. Yet when team members played one another, the highest-ranking males tended to produce less testosterone than their defeated teammates.

I can actually support this with my own personal study at the University of My House. I have a weekly LAN gathering among friends and it's a lot less stressful than playing against strangers online. Here's how I'd rank the levels of stress, which I presume is a factor of all that testosterone coursing through my manly veins:

#1: Beating strangers: "God, that was awesome and I could use a cigarette about now!"
#2: Being beaten by strangers: "I hate this game and never want to play it again and now I need to calm down by taking a bubble bath or something."
#3: Being beaten by my friends: "Wait, he won? Whatever..."
#4: Beating my friends: "Oh, I won? Huh. Okay, fine. Let's go again, I guess."

Here's the actual study in case my data aren't good enough for you.

(Thanks Slashdot.)