Even before a game begins, an athlete’s body changes: heart rate increases, hormones surge and beads of sweat dapple the skin. Competition is such a visceral experience that the mere anticipation of a challenge excites our instincts to fight. These biological responses are even more pronounced when people face an opponent they have come to know and despise, an opponent they must battle again and again—a rival. In a 2003 study psychologists at Northumbria University in England found much higher testosterone levels in soccer players preparing to play against a team they considered an extreme rival than in those matched up with a moderate rival.