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.
"In some instances, highly identified fans may also view it as a means of helping their team," says End.
Leonard Zaichkowsky, Ph.D., a professor of education at Boston University and an expert in fan behavior, says it's unclear why American fans are jumping into the game.
"Is it the violence in sports that motivates the fan or is it a microcosm of what is going on in the world?" asks Zaichkowsky. Researchers say they will probably need a few more seasons to figure that out.