An anonymous reader writes: To study how aggression, fighting, and winning change the brain, scientists set up a tournament of mice fights. They watched as the lab rodents took a break from their hum-drum existence and battled it out (however, the researchers broke the first rule of Fight Club by publishing a paper about their findings). They found clear evidence of the "winner effect," in which a mouse that has just won a fight maintains elevated levels of testosterone and aggression, and is therefore more likely to win the next bout. Interestingly, the winner effect was strongest in mice that were fighting in their own cages--i.e. those that had home-field advantage.