On a warm night in 2003 at U.S. Cellular Field in Chicago, a White Sox fan rushed the field and tackled the umpire. Four days later, a right fielder with the Texas Rangers was hit in the back of the head by a spectator's cell phone.
Fans who become part of the game are what researchers call "high-identifying sports fans"—people whose identity is intertwined with a team. High-identifiers tend to have extreme emotions in the face of defeat, compared with average sports fans.
Researchers have found a wide range of personality and social factors that influence the actions of high-identifiers. Alcohol often encourages their behavior, as does the possibility of appearing on television, says Christian End, Ph.D., assistant professor of psychology at the University of Missouri at Rolla.